I have a Wireguard VPN to my home network and there it works exactly like you tried it. Connect to VPN, then use the internal IP to connect to the service. Maybe there are settings in the server or client that prevent that. For example, you can set specific IP ranges to not be routed through the VPN (so you can access a local service in Network A even though you are connected to VPN B).

I’m no network expert, but maybe some additional information about your setup can help other user help you better.

I left reddit because of asshole decisions. I think 18 Euro per year is pretty fair for an app you probably use every day. I prefer one time payment over subscription, too. But the price for sync is nothing to rant about imo.

An IQ of nearly 300 seems pretty galaxy brain imo.