For anyone who is still getting this problem - as I did with my
mini 2, I think I have found the cause and the solution.
The message Range/height restricted to 50/30m sign in with real name etc. Was a red herring in my case.
The problem, I think, was that the mini sometimes needs several minutes to find sufficient GPS satellites for an accurate fix. When it only has a few satellites, it will take off but range and altitude will be restricted to 50/30m.
So what I do now is reverse the setup procedure advised by DJI Fly. I,e......
1) extend the rotors on the mini, switch it on and position it ready for takeoff
2) switch on the remote control (which then starts communicating with the mini).
3) connect the remote to the mobile
4) turn on the phone and open the DGI Fly app.
You should find that the app immediately opens in camera mode.
The app should immediately confirm that home location has been updated
Check that the GPS symbol is white
Follow any calibration requests.
Set RTH height etc
By now the mini has had several minutes to establish a strong GPS fix and you should find no height or range restrictions other than those that you have set in 'control' or that are determined by your geolocation.
This has worked for me with no issues every time so far. I hope it works for others who are experiencing this frustrating issue.