Update: I have managed to connect to the internet using the EIC modem and a ZyXel router (NBG4604) so am now wondering if there is an incompatibility between the USG and the modem. As mentioned before, the USG says it is connected to the internet but I cannot PING any external IP address. I get the message "PING: transmit failed. General failure". I have tried flushing the DNS and renewing the IP address using ipconfig but have the same issue.

Did you get a UI controller up and running and adopt the USG on it, to check the firewall etc settings on the USG?

No - I connected a laptop to the internet using my existing network, got a UniFi controller running in a browser window (so that Ubiquiti was able to authenticate my account), disconnected from the existing network and connected to the new network. The USG is there pending adoption, but when I tried to adopt it, I got the message "adoption failed". Same thing happened when I tried to adopt the WAP on the new network.

I was able to look at some of the settings of the USG via the UniFi controller but there is nothing there that I can't set by connecting directly to it via a laptop and the USG IP address. There are no firewall settings on the USG

The symptoms you describe - USG apparently connected to internet, but anything behind it, unable to access internet etc. smacks of a firewall / routing misconfiguration. If there was an incompatibility between the USG and the modem then the USG wouldn’t be saying it’s connected to the internet.

The USG will need to be adopted into an active and internet connected UI controller (ie online) for you to configure it.

Without having one here to confirm, I believe that you will need to setup some basic firewall rules.

Hi pheasant - thanks for all your help and patience. I just don't know how I am going to sort this out .... For example, I have no idea what firewall rules to set. I connected the USG to the existing network and fired up a UniFi controller in a web page but it can't see any devices to adopt - not even the UniFi WAP which is happily giving access to various devices on the existing network. I can't even PNG the USG when it is connected to the existing network! The existing network is 192.168.0.x and the USG 192.168.1.1 so don't know if this is an issue. Completely out of my depth here ... sigh ....

OK. Getting a better picture now. For starters it looks like you have some DHCP related addressing issues to resolve

Presumably the addresses on your existing network are being given out by a DHCP server which is resident on the Zyxel (the .0.x range)? This is why you cant necessarily ping the USG when you connect it on your existing network.

You would then need to change the network mask on the DHCP server on the Zyxel so that it can 'see' the default subnet of the .1.x network the USG is set to.

Have you managed to get your router into bridge mode. If not you will have Double NAT problems.
Also if you can't get the USG to adopt you may find this link helpful Unifi network add new device

With my USG I had an old bt modem that I could use to get rid of the router to avoid double NAT problems.

Once you get the unifi setup working and have a chromecast or apple devices I believe, I found this very helpful in setting up the firewall rules reddit link to firewall rules

I found getting it to work with the router in the way a real pain until I remembered the old setup I had where plusnet sent out a modem and a router so dug out the old modem before going full fibre so I could just use the USG straight to the internet. If I want more then 100Mbps then think I will need to swap out the USG for something more capable.

It was a Huawei HG612 VDSL / FTTC Fibre Modem but believe Draytek Vigor 130 is similar

Hi TAZZ69 and Pheasant
after your last posts I have spent more time fiddling round with the network. After writing in my last post that the new devices could not be adopted, I turned the new network off, turned it back on again and connected it to an old laptop running Win 7. When I fired up the UniFi controller (v4.48), all the devices had been adopted and their lights were blue. Hurrah! I tried connecting to the Internet via my old EIC modem (similar to TAZZ69's Huawei) but was still unable to connect. But progress of sorts. I then thought I had better upgrade the UniFi controller on the laptop to Win 7 to the latest version. After a lot of messing about, I finally have UniFi controller 6 running on the laptop and connected to the new network. I have factory reset all the devices on the new network to "unadopt" them and their lights are all now white. However, none of them are available for adoption on the UniFi controller. I have opened TCP port 8080 and UDP port 3487 as suggested in the link TAZZ69 sent (and turned off windows firewall) but to no avail. I can PING all the devices from my laptop but the UniFi controller does not see them so back to square 1 ....

Did you tell the controller to forget the devices? if not then they cannot be re-adopted until this happens as they may still be present in the controller settings. - I had this issue yesterday adding a new AP that initially failed adoption.

What shows up in the devices tab in the controller?

What settings do you have under WAN in the Internet section? It should be PPOE and your username/password - there should be nothing else to set and should just work.

Do you have a static IP on the laptop that is on the same network as if that has changed then the controller and USG are not able to "talk".

There are some other settings etc. to change depending upon your security requirements, DDNS, BQM (need to enable ICMP pings) etc. but basic authentication and throughput should be simple. I used to run USG with the controller on a Pi 2 and it was all pretty straight forward.

Hi NGDragon

please see below

Did you tell the controller to forget the devices? if not then they cannot be re-adopted until this happens as they may still be present in the controller settings.

No. I had to do a refresh install of the UniFi controller - trying to upgrade from v4 to v6 caused the controller to hang on start up, so I had to delete the previous settings (and forgot to make a back up ...). Previously I logged into the controller using a user name and pwd - now I have to log in using my Ubiquiti email and pwd (which are different) - but have absolutely no clue how the two are related ...

What shows up in the devices tab in the controller?

Nothing ... completely empty apart from saying "No UniFi devices have been adopted"

What settings do you have under WAN in the Internet section? It should be PPOE and your username/password

That's it - but can't connect to WWW via the modem even though the un and pwd have been accepted. This is another issue that I still have to resolve

Do you have a static IP on the laptop that is on the same network as if that has changed then the controller and USG

No. The laptop IP address is set by the DHCP on the USG. I am logged into the USG directly from the laptop and the laptop is showing as one of the devices connected to the USG with the same IP address that ipconfig gives on the laptop

I used to run USG with the controller on a Pi 2 and it was all pretty straight forward.

That is what I would like to do too and I was hoping it would be straightforward but it has been anything but so far ... sigh