how to set up sync directories?

Hi, I’m trying to setup a sync between my qnap nas (v1.12) and my android phone (syncthing-fork). My problem is that when I first set a “a” directory to sync on device “A”, when I sync “B” device I cannot choose the local directory, the software chooses his own. Starting on the “B” device I ended up sharing a “b” directory on an arbitrary directory on “A”. At the end I got two sinc’d directories wich I can’t find on the devices. How do I set in sync b directory on B device with a directory on A device? Thank you