Riiiiight, thanks for the hint! I didn't see that at all! Though, configuring Cloudflare, Google, Dismail and other DNS services didn't make it work either.
Thus, I tried installing AdGuard Home on a USB drive plugged into the router (including resetting everything beforehand), but unfortunately, it hasn't worked yet. I've already posted my problem on the “Installing AdGuardHome on OpenWrt” thread: