When connected through wireguard can you access anything on the local network?
Also sometimes if you're on an LAN trying to vpn into itself you get weird network issues. Does this issue also happen when you're on another network and vpning back?
Also there's a lot of great videos on YouTube to set this up things like Lawrence systems and network Chuck.
Not exactly sure what the issue is here but gave you the up vote for visability and hopefully someone smarter then me will know.
Unfortunately it seems like the dev has abandoned his app. I also used Sync for reddit and was happy to pay for the Lemmy version. Money down the drain. At best it seems that there will be updates every 18 to 24 months, maybe if at all. This may have been fine on reddit but Lemmy is constantly being updated and changing. So the app is more often then not broken or missing functionality in some.way. Honest answer is to find another app and donate to another developer.