I am using a Draytek 2760 and I have this as my DHCP server and the pihole on a raspberry pi formerly on a debian VM , my internal DNS does not resolve to an IP address, in short I can not RDP to other machines in the home unless I enter the IP address not a major headache but would be nice to get it sorted, I think it is something to do with the conditional forwarding option , see image
This change has not made any difference when trying to RDP to a machine in the home
This will lead to issues as your clients pick the DNS server they want. Google does not know your internal DNS records which might be the reason the resolution does not work.
You should remove the google DNS from your router's DHCP advertisement, re-connect the clients to your network to pickup the new infos and try again.