It's fixed. Used a Dell adapter instead which supports higher resolution.
I'm using the HDMI cable that came with the S3422DWG with my laptop Dell Precision 5770 using an Anker dongle. I can't go higher than 1920x1080 when choosing from Windows 10 settings. I've tried to update the driver and installed DDM (Dell Display Manager) from Dell's website. No luck! 
S3422DWG, Precision 5770, Anker dongle, no higher than 1920x1080?