If your phone is in “Hotspot” mode, you might not be using the correct IP addresses. I use an app from Google Play on my phone called “IP Tools: Wifi Analyzer” to determine my cell phone’s hotspot IP address.
For example, right now the internal Hotspot IP address of my cell phone is 192.168.216.199, and it handed out 192.168.216.5 to my laptop.
This is the networking information I would need to use in syncthing to make a manual connection to a specific IP address in this scenario.
Hope this helps.