If your router doesn't allow you to specify your own DNS then there are a couple of options.
Replace your ISP provided router with something else if possible, the BT homehub 5s can be flashed with OpenWRT and they are much more configurable than an ISP provided router. You can even buy them ready flashed with WRT from ebay for about £20 if you don't want the hassle of doing the flashing yourself.
Set up your own DOH or DOT DNS server on a cheap VPS to encrypt your DNS traffic from your devices so you are bypassing the routers DNS settings. You could even run Pihole on it and block ads as well
Biting the hand that feeds IT
