Update: Just turned off AVG Internet Security running on the laptop and now all devices are visible!! Just need to get the USG to connect to the WWW and we're there .....

Is the USG connected the modem (Huawei or ECI?) in the same exact manner as the successfully connecting router?

No - I managed to get the ZyXel modem connected but as a regular modem/router not in bridge mode

Have you got the Zyxel setup doing the PPPoE then….with the USG downstream (also trying to do the same)?

Unfortunately not ... it is not possible to put the ZyXel in bridge mode. I have a Technicolor router that is currently providing internet access to the old network and which is (theoretically) possible to put in bridge mode. Unfortunately, that did not work either so the only option at present is the ECI ?EIC? modem. Am thinking about getting another modem ...

Ah sorry, misread your post ... no, I have not had the USG and ZyXel both connected at the same time (and both trying to do DHCP)

Have you checked that PPPoE works correctly with another router (or even your laptop with PPPoE client) with the ECI modem?

Yes - I have tried that using the laptop WAN miniport but the connection failed. I have tried it with another laptop running Win 10 and that seemed to produce BSODs so it could well be an issue with the ECI modem

In reply to a post by pet_griffin:

Yes - I have tried that using the laptop WAN miniport but the connection failed. I have tried it with another laptop running Win 10 and that seemed to produce BSODs so it could well be an issue with the ECI modem
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This all sounds complex, is it really needed?

Shouldn’t really be a big deal. Admit I haven’t done it for ages on windows, but setting up a PPPoE connection on something like MacOS is dead simple, takes maybe 20 seconds - a handy way to confirm your PPPoE creds etc are working correctly or running tests without having any other routers etc in place as you’re just connected directly to the modem or ONT.