Hello,
I missed your message the first round when I was away.
The resolution offered is the maximum of HDMI.
I believe the DP->HDMI adapter you are using is a passive adapter. Therefore it uses the DP++ feature which is, in simple terms, HDMI signals over the DP plug.
https://en.wikipedia.org/wiki/DisplayPort#Dual-mode
I suggest you get an
active DP -> HDMI adapter so what is going through is actually DisplayPort and not HDMI.
Kind regards,
Alban