Because I still had access (through Tailscale) to the RUT360, I ASSUMED that everything was working, but it wasn’t.
Just now I have gone down a rabbit hole of trying to understand why IPv6 doesn’t work on EE/BT and the conclusion I have come to is that it just isn’t possible - I haven’t come across anyone doing it successfully.
Which is a shame, as I was hoping it would mean Tailscale could operate without a relay.
However, ensure that your internet service provider also supports IPv6. Sometimes, you may need a different APN to enable this functionality, as IPv6 isn’t widely used in Europe.
Thank you Marijus - I did try IPv6, and the RUT360 connected fine and was given an IPv6 address by EE, which worked for IPv6 but then everything on the LAN using IPv4 seemed to fail. (And the router lost connection completely when I selected the ‘IPv4/IPv6’ option.)