I have found the issue about syncthing writing to external sd Writing to external SD card in Android 5.0 and above But shouldn’t it be possible to read from external sds i.e. set folder master check box. Is there a workaround for this?
You need to create a .stfolder file at the root of the directory you are syncing for syncthing to consider it readable.
If worked actually.
Here is the test that I did: I opened syncthing-android and using “Advanced folder picker” selected a folder on the extsdcard (although it showed a message complaining that my android version does not support syncing to extsdcard) and created a sync folder shared with a PC and “folder master” checked. On the PC I accepted and created the corresponding sync folder. After restarting syncthing on the phone it showed that folder in “error” state and nothing was transferred to the PC. So I opened WebGUI on the phone and manually stopped syncthing. Then with the phones default file manager (it does not support creation of empty files) I copied an .stfolder file from another sync folder to the new one on extsdcard and restarted syncthing. And voila - everything works as expected. The folder syncs just fine, copying files to it sends them immediately to PC.
Thanks for the hint, AudriusButkevicius 
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