a thoughtful use of digital
The best android open source Firewall (+ adblocker / vpn ) I found so far is Rethink. It can have some problems (i.g., recently, with Whatsapp or Fastnews, blocking them from the Net), but you can solve them quite easily.
Not so good I tried apps such as Athena, or De1984, or InviZible Pro: all very difficult to set properly. Indeed they block either too much or to little (your apps from the Net).
Rethink, on the contrary, can let you set exactly what should be blocked and what allowed.
If you are not able to connect your PC to your mobile device, using Syncthing in both of them, you should check your firewall, both on PC and on mobile.
And, of course you have to set, in firewalls, permissions to communicate each other.
In this case, as in many other, it a good idea to backup your settings. Indeed if you do a hard reset of your router, you loose your settings.
To backup settings you should use a browser (web-like) interface (firefox, or chrome, or an other one) using a url like http://192.168.1.1/ or something similar, and search for “backup” (or “save”) router settings.