I have multiple devices in different locations. These are laptops and mobile phones. Syncthing works via UPnP on a router, correct? Where I connect to Wifi, I never have physical access to routes, no access to setup UPnP on them or whatever is required to setup syncthing.
And I often use Wifi in different places. Sometimes I only have mobile internet.
Yes, worst case you will connect via relays. If there is just one device that can get a port, connections to that will work (e.g. if you have one device at home/own workplace). I have no clue how mobile internet works with regards to routing.
In my experience it works really well. I’m often on mobile Internet and have no problems connecting via relay to those devices that require it, and directly where I’m able to.