IMac 2013 version Retina not available


I am using
IMac 2013 2k
Luna display mDp
IPad Pro2 12.9.

I can not use the Luna Display Retina setting in this case, and I am disappointed that it is fixed at the low resolution setting.

The AstroPad can be used to set the screen area freely, but I can not understand that the Luna display does not provide this option.

I would like to ask if I plan to add a resolution change option when working with iMac 2013 and non-Retina mac.