It might be BT kicking you off, I assume you're using FTTP with an ONT on the wall? if so the installs sometimes don't play nice with 3rd party equipment, I'd ask BT to do a line check first so see when it happens, also check the usual suspects 1 - Too hot a router so place it where it can breath better & 2 - wifi interference so make sure it's away from microwaves and other wireless devices you may have in the home, also check it's not something like a cctv camera or a search assistant product causing it assuming you use either, this is process of elimination so start testing by removing one device at a time from your network, if it stops terminating the connection you've found your culprit, hth.

  @James335 

Cheers for coming back to say what fixed it, I did suggest a different cable in another persons thread but forgot to include it here as a possibility, glad you got it sorted, sometimes the cable doesn't always connect fully as well (at either end) or a certain LAN port for whatever reason doesn't activate but will after a re-boot, there's loads of silly reasons people get problems with & the best way is to always rule out the simple things first.