read elsewhere that local discovery was only supported on rooted android devices, so to have my phone device discovered by syncthing on my machine running windows 10 I had at least temporary check "global discovery" on each of the participating devices.
My default configuration though sets them to false (https://docs.syncthing.net/users/faq.html#should-i-keep-my-device-ids-secret), temporary meaning manually on demand when I actually want to sync.
At the same time I keep "NAT traversal" (https://forum.syncthing.net/t/enable-nat-traversal-what-does-it-do/13044/4) disabled as it is not needed for my purposes.