4k and 2k in a 1080p, help

I know that DSR is a thing, but:

My BenQ Mobiuz 2710S (HDMI 2.0 and Display Port 1.2 connections) came with 1 HDMI 2.0 cable and i bought a 1.4 Display port cable.

When i connect the HDMI cable, if i go in game (without messing with DSR), i have many diferent resolutions that go above my monitor native resolution (1080p 165hz), i have an option for 1440p 60hz for example. But when i connect the Display Port cable that i bought those options disapear and i need to turn on DSR.

What are those options that only apear with HDMI and should i use the Display Port 1.4 or the HDMI 2.0?

And with DSR should i do the scale on the gpu or on display? Aspect Ratio, Full Screen or No Scaling, do this settings affect how DSR works?