My Samsung Dex (with Android 7 in a Samsung Galaxy S8 phone) won't work with my LG 23MT75D-PU monitor. I think because the Android driver are not ready to talk with this monitor yet. I will want to know where i can fire a bug related with that fact or find a fix, by myself.
I really think this is an android question by nature. My Galaxy S8 have not a problem and also i considered, this is not a problem of Samsung Dex, but instead, this is a problem in the Android kernel, because it's where reside the modules to load the most commons drivers by default.