Accessing Synology NAS via FQDN

I have a Synology NAS on my network. When I am outside the network connect to the VPN via wireguard/Rasberry Pi 4 I have to use the IP address and not the FQDN to access my network share. Any ideas why? I have no issues when at home using the FQDN to access the network share.

You need the DNS to work outside the network for that to work, or make sure your system is using the internal DNS when on the VPN.