Try turning off the router, and modem if separate, then unplug at both ends including the phone socket(s) all signal and power cables to do with them. Then reconnect them before powering up again. If you have a dangly filter at the phone socket end, take that out and re-insert it as well.
If still as bad, or even if it clears the problem, then gently waggle the cables about and flex them to see if that makes any (even short) difference - testing if any are damaged inside.
Kindness isn't going to cure the world of all its awfulness but it's a good place to begin. Daisy Ridley.
My broadband basic info/help site - www.robertos.me.uk.
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