How often are the reboots? It's not that unusual for domestic routers to reboot every several weeks for seemingly no apparent reason.
It could be you have some sort of denial of service attack happening, there are several that will cause routers to reboot, are you on a static IP address? If yes, it might be worth trying to get a change of IP address, if no, are you receiving the same IP address each time you connect, again if yes, you need to see if you can force a new IP address.
When you were on an Openreach Modem + Router, was it only the router rebooting in that case? If Yes, rules out anything interference I should think coming down your telephone line as it would have affected the modem.
I think you can rule out power issues given you have tried a UPS, and the switch mode power supplies used are pretty forgiving to brownouts etc anyway, and will often work quite happily at lower voltages and care little for any mains Hz frequency variation, this is why most power bricks work internationally (from 120v 60Hz upto 240v 50Hz) without any changes to settings etc.
To confirm, when you say reboots, do you mean the line is dropping and re-syncing or do you mean that the router is crashing and rebooting from scratch?
Edited by PhilipD (Thu 24-Jan-19 10:51:28)