Sounds like the service provider just turned you off for a bit, accidentally or on purpose
They turned the service on and off a few times while they were testing. Anyway, I'll be leaving the current iffy router on 24/7 until the new one arrives. It's due tomorrow PM.
The new router arrived yesterday. It's now connected and has configured itself without any input from me. As the old one has decided it now wants to work, I'll leave it on the other PC. Sorted!