Well, i have also problems with DDC/CI with my sapphire rx 570 nitro+, i found something like this, i think some pins are responsible for DDC data transfer, i am not sure, but if things are like on this photo, maybe your display port cable has no DDC pins on it, or your graphic card doesn't, I am still also trying to find out how to solve my issue, i wrote finally to sapphire, btw you will understand what i mean by this photo, i guess the pins responsible for DDC are as on this photo...
https://www.datapro.net/images/techinfo/hot_plug_detection.jpg