I think is something with the app, I just disconnected from the modified mi fit and connect on the Gadgetbridge and worked lol, now that I'm back to mi fit is no longer working.
I do everything you say and didn't worked :(
I just tried to synced 4 times and shows the same message.
Edit: and yes, mifit app have all permissions allowed
I'm using English firmware, just installed your app, put my city (Contagem - Brazil) in the weather option but in Mi Band 3 shows "Connect to the app".