Thanks. I read the links but still havent succeeded. This one seems definitely relevant: Is it possible to use Syncthing with no internet access? And is it possible to have all changes go through a single server?
The phone (which has only Tailscale tunnel access, no access to internet) wont even load the Syncthing app without hitting the server.
I tried to out the Tailscale IP and connect the device from the Umbrel server but nothing happened, the device never connects.