Any other devices using WiFi or 2.4GHz band: TV, Set top Box, Sky TV, TV home theatre, security cameras, Blue tooth devices, wireless keyboards and mice ?
Hm, you reminded me there is a Blu Ray player. Our TV at the moment is not smart, Sky got ditched years ago and the only 2 bluetooth devices we have are relatively recent; so unlikely to be the cause. No wireless mice or similar; used to have them but found the wired ones performed better. Security camera is another recent thing, got it well after the problems with previous routers and well before this one started getting dicky.