I had similar issues with my BT router (a good few years old now). After much faffing around and investigation I came to the conclusion that it was due to interference with other WiFi networks I was in range of (I could see another 6 or more). So I decided at the time I needed a dual band router so I could transmit my WiFi on 5GHz rather than 2.5 (or whatever it is) GHz that is the usual standard assuming most other people were on the 2.5ish GHz frequency. The only downside appeared to be that 5GHz can’t penetrate walls as well, but since I have a newish house with plasterboard internal walls I didn’t think this would be a problem. At that time there were not many dual band routers around, but the Apple Airport Extreme was one, so I got one off eBay. I turned off the WiFi side of my BT router so was just using it as a modem, Plugged in the Airport extreme, in Typical Apple stye within 30 seconds after a quick firmware update it was up and running and it has been ever since over the last 4 or so years and hasn’t dropped out once. It has literally been 100% reliable. I’ve got an Apple Airport Express and a wireless range extender which also seems to work fine too.
I’m no networking expert so cannot explain why this solution has worked for me and i’ve probably just stumbled at random on a solution, but it’s worked a treat.