Does anyone have such issues?
Apparently now I have to use the navigation bar in order to do a back and open up menus via screen, instead of the button situated on the left and right of the fingerprint sensors.
Hi
The physical buttons will not work if you have virtual buttons activated.
If changing of physical /virtual settings are correctly set and there is no help you should perform factory reset.
with virtual settings switched off, the return button does not work, neither does the other one works as well.
let me try a factory reset, and see if it works.
As probably you have tried every possible option which user can do (checked /change settings, factory reset) so you should contact seller.
Of course you can try SPFT and flash system to latest one to overwrite all partitions/drivers.
Here is SPFT updating instruction http://community.umidigi.com/forum.php?mod=viewthread&tid=11
And here you can find all available ROMs http://community.umidigi.com/forum-131-1.html
I would try android 6.0 firstly.