I just want to report back to the communicty after a week or so of issues. I got a new Pixel 6 straight from Google (upgrading from a Pixel 4a 5g). I had been using that phone daily to tether my laptop (and enjoy some online gaming etc) for months. After swapping the sim into the new phone I encountered significant connection issues (couldnt connect to the gaming server. Massive amounts of dropouts. Problems connecting to Steam). Assuming it was the gaming service, I went round the houses with them, only for them to finally conclude it was my ISP. On the phone to EE they weren't very helpful, and could only suggest that it was because of the network type (CGNAT), not supporting the game. This ignored the fact that I'd been playing on the Pixel 4a 5g for months on the same sim. Anyway. I won't go into the long sorry tale of the different things I tested. But this thread helped. Creating a new APN and turning off ipv6 has (seemingly) sorted out the issue.
Just thought I would post this incase anyone else hits a similar issue. tyvm
