Your phone gas the FM radio chip but is disabled by default (as the clone of m3 note: the Yu Yunicorn)
So is very difficut to re-enable It, but i tell you how.
The FM radio chip is the MT6627, so we need to add some files to the rom: https://drive.google.com/folderview?id=16447rXV2XOmzt7TVrHivdJz0K95W17br
After we need to enable FM radio drivers in system/build.prop modifing a lunedì from fmradio.driver.enable=0 to fmradio.driver.enable=1.
At the end we have to find and  install the FM radio system app.
Why we don't create a development team with also a telegram channel for the M3 note even L681H?

so complicated