The OP states they have full fibre , if that’s the case then checking the ONT during these periods is necessary to try to determine if it’s an Openreach issue affecting BT or just a BT one , although they also say Openreach say the network is fine (although how the were told that isn’t mentioned) .
FWIW , my SH2 on FTTC has started dropping out and restarting probably around every two days , no specific time of day , it just drops and comes back up randomly, it’s a pain but not sufficiently so to call and complain about it .
