If this was a fibre->fibre transfer the fibre (VDSL, cabinet->modem) connection should not have been broken at all.
It will only be the higher level connection that may get broken.
Plusnet should set this up automatically within 1/2 hour or so of losing BT;
but sometimes that fails and you need to contact them.
Once you finally get through they will set it up instantly.
The router needs to have the right Plusnet credentials for login before it will work.
You can put those into your old BT hub (as I did for my entire stay with Plusnet), or plug in the new Plusnet router.
Moved from PlusNet back to BT. Both gave decent service.