If Syncthing is able to establish a direct connection between the two machines (meaning that it is able to find the IP of the other machine (through local discovery using broadcast packets, a discovery server, or hard-coded IP addresses), and it is able to get through firewalls), then it will do so. Otherwise it will fall back to using relays.
My guess is that the local discovery packets aren’t being routed between your subnets, so Syncthing is unable to figure out the address of the other server. You could hard-code the device addresses, or run your own discovery server inside your network.