I assume that you want the two to connect without depending either on global discovery or relaying? Because normally Syncthing is supposed to connect just fine even if the devices are located in completely different networks.
As phone has been mentioned, which I’m assuming is Android, please keep in mind that local discovery is broken in newer version of the OS. You need to have at least global discovery or relaying enabled for it to connect (unless you hard-code the IP addresses or hostnames).
You need to hard-code IP addresses of the remote devices. This can be done in the “Edit Device” pop-up for each device separately. The default is dynamic. There’s a tip right next to it on how to include specific IP addresses, and if it’s still unclear, please check the Docs as all the information should be present there in detail as well.