If you have FTTP then the status of the ONT lights are informative, is the PON light green and steady or is the LOS (red) light lit , LOS = loss of signal, would indicate a physical problem with the Openreach fibre , upto it entering the ONT , if the PON light is steady then the OR network is fine with either case could be the ONT itself (but that’s not very likely) , but switching it off and on and ensuring the green plug is fully ‘in’ is worthwhile.
If the PON light is steady green , then it’s going to be a BT issue , but even if it’s LOS that’s lit , you still need to call BT to report the problem irrespective of where the issue is
