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.
KDE Connect not working
You have both smartphone and PC connected at the same network. But no connction between the two.
A possible reason is a firewall on the smartphone, .i.g. ReThink. In this case you should exclude KDE connect from firewall.