Extended display on Dell SE2717H not works.

I have an iMac 27"  Mid.2010 with graphics adapter AMD HD 5800.

Extended monitor function with Dell SE2717H works fine with macOS High Sierra but when I switch to Windows 10, only mirror display is working.

Once I select extended display, resolution is set to 640x480 and can not be changed. Please see below photo.

I have installed Windows 10 Pro 64 bit. I tried to update/reinstall the driver of graphics adapter but the problem remains. 

Any support it would be appreciated.

Many thanks

ACK