It’s possible that the app has been disabled on the Play store if the developer has failed to raise the SDK level. A bunch of apps have been delisted for that reason.
If you still have the purchase email you may be able to contact the developer about it.
Syncthing like others suggested is probably the way to go.
But if you want more options you can do a lot with WebDAV. https://www.digitalocean.com/community/tutorials/how-to-configure-webdav-access-with-apache-on-ubuntu-18-04