It's very much time specific. Absolutely fine in the morning, but in the evening it keeps losing connection to game servers. I'm going to try some other multiplayer games over the weekend and see if it's specific to this game.
It's on PC rather than console but that would make sense. The VPN would mean the router just saw a 443 TCP Web connection tunnelling from the PC to VPN servers and it would skip all its Nat on any other ports used by the game in and out.
Maybe I'll disconnect everything else and run the pc through it in modem mode and see how it works. I don't want to do anything that exposes less secure devices (TV, light bulbs, IoT bits and bobs) to the wider Internet.