RUT951 blocking Smart plugs Echo dot from internet

I recently replaced an ageing and failing RUT950 with the RUT951.

Most of my devices worked fine but it seems to be blocking my Echo Dot and, I think, my smart plugs (Meross and Tapo) from accessing the internet.

As with my old RUT950, I have a wired ethernet connection to three BT Whole Home mesh routers and all my devices connect to the mesh router wifi which provides both 2.4 and 5Ghz.

I have been unable to get my Echo dot (or a separate new one) to connect even though it does connect to my mesh wifi. It did connect correctly when I used a borrowed TP Link router instead of the RUT951.

I now have a Google nest mini, which has connected OK but shows all my Smartplugs as offline.
I can see and operate the Smartplugs correctly through their Meross and Tapo apps but I can’t connect them to Google home because they show up as offline to Google home.

I installed a new TAPO Smart plug which works successfully with the TAPO app but is flagged in TAPO as being offline and suggests that I try changing the DNS server on the router to 8.8.8.8.

Another issue that I’m guessing is related, is that both of my Amazon Firesticks work OK with the new router, except when I try to access Apple TV, it fails, stating it cannot connect to the internet. iPlayer, ITVX etc work fine on the Firesticks and I can access Apple TV through my iPad and Windows laptop (via the mesh router wifi, and then the RUT951 router).

I have updated the RUT951 firmware to RT9M_R_00.07.10.2. The internal modem firmware is EC25ECGAR06A15M1G_20.200.20.200 and the kernel is version is 5.15.165.

I have the same issue if I turn off the 5Ghz band on my mesh wifi before attempting to connect, or if I connect directly to the RUT951 wifi.

I would be grateful for any suggestions to resolve this. I haven’t yet tried the DNS server setting as suggested by Tapo, partly because I don’t understand the implications of doing that, nor do I know how to do it. I would try it if it looks like it is appropriate.