If it said it couldn’t resolve, yeah, it was a DNS issue. Usually the DNS conf in wireguard is to overwrite the local DNS on your computer with the DNS from the remote private network. That is used mostly when you have private IPs that you need resolved there. You most likely know the IPs on the Pi and the Debian server, so you can just place an entry in /etc/hosts and be done. Yeah, DNS setting in wg conf is not needed in this case.