Syncthing does not get any connection as long as AFwall+ is activated. It should work in my WLAN at home. Mobile data is disabled because I don't need it. I am using LineageOS on my mobile phone.
The solution was to disable wireless (WLAN) in AFwall+ and activate LAN.
This does not make sense for me, but it works...
It still works if I activate wireless as well, but it is not necessary.
Even if I disable wireless access within AFwall+ I still need to be connected to a WLAN within Android (what makes at least some sense).