I bought MR600 few weeks ago.
I'm using Airtel simcard which supports 4g+.
I also installed beta firmware to manually select frequency bands(in my case band 3, 40).
When I hit save it connects to 4g+ and speed was amazing.
After some period of time, it automatically switches back to 4g(in my case it switches back to band 40 only).
When I check the band selection it still on band 3, 40.
Now I need to set both band to 3 and then change one band again to 40 in order to get 4g+.
I did checked my signal strength it was really good.
I don't know why it automatically switches back to 4g even 4g+ available.
Expecting a quick support