Gotcha ✔️

Yup, I second NextDNS. It works perfectly, and the free tier is enough for my mobile, wife’s mobile and my desktop. AdGuard is included, as are dozens of other (selectable) lists.

In that case you totally don’t. But many ISPs only allow their own routers/modems or require some very specific abilities from 3rd-party routers usually only found on more high end (expensive) models. So sometimes the last resort is double NATing (which is fine for most users).