The laptop uses the same WIFI as the Android device.
The PC and the NAS on which Syncthing runs in a docker container use LAN.
The laptop and PC can connect and sync with the smartphone perfectly fine, only the NAS Syncthing can't connect nor sync with the phone, nor when the phone is connected to Wi-Fi nor when its on cellular network...
I also double checked the smartphone so that syncthing is not battery restricted or anything and also reinstalled the app once to make sure.
But as it works with smartphone <> PC and smartphone <> laptop i don't think the phones app is the issue.
After reading this information, the only thing that occurs to me is a possible configuration error between the connections of some nodes with others.
I'm sorry but I don't think I'm going to be able to help much more with this. If no one contributes something you would get better support on the Syncthing forum. https://forum.syncthing.net/ They know your application better than anyone, of course.