When you plug the ECI modem into the phone line and turn it on...what lights do you get?

All green. I have had the modem connected to the WAN and DNS servers set from the WAN side but I can't access the internet - can't even PING the DNS servers

Are you sure you have the correct details that you are putting into the controller, the important one is the VLAN ID
If you let us know who your broadband supplier is I'm sure we can figure it out, either that or the modem is not working correctly.
I'm sure you have the correct details username and password as that must be in your current router, I recently switched supplier and they were not great at telling me the correct VLAN ID (they gave me the wrong one, luckily I had done a bit of research so had a good idea what it should have been)

In reply to a post by Pheasant:

Admit I haven’t done it for ages on windows,

TP Link has a handy page, but Winkey+X no long has control panel in the list in later updates.
https://www.tp-link.com/uk/support/faq/921/

If you have an FTTP connection, or are using a third-party FTTC modem, you may have to configure a VLAN. However with an Openreach-supplied Huawei or ECI modem you should not as the VLAN 101 tagging is already done by the modem.

Does the PPPoE connection on the USG connect successfully and show a valid IP?

Which ISP are you using?

Hi have tried connecting the USG both with and without a VLAN (101). With no VLAN set, the USG makes a connection but is not able to access the internet; wth VLAN set then the USG is not able to make a connection

Hi tdw42 . yes, the USG will make a PPPoE connection and has an IP address and DNS servers set by the WAN but that's as far as it goes. Just wondering now if this is an issue with AVG firewall - that was stopping me from seeing the UI network until I set the network to be "trusted". Let me try that over the Xmas hols ...

You have had some with more experience/capability than me helping so far, such as @Pheasant, so I have said nothing.

I had a USG working through an OR modem - although it may have been a Huawei HG612. I cannot remember any major issue setting it up. I will check to see how it is configured but I believe that I may have changed it to PPPoE when I moved to FTTP.

I’d just completely uninstall all that third party firewall junk. It’s honestly more trouble that it’s worth. The built in gear is all you need.