Final final test for the monitor: the USB problem is partly due to the USB cable that came with the device. Not sure what's going on. The cable seemed to work and charge the phone, but it would not carry the picture display on the phone like it did in the first few days of trying. Apparently, I tried several other cables at the time, but none of them worked.
I purchased a new USB to USB-C (v3.1!) cable and it does work!
To use the Samsung Dex via the USB port, the display requires an additional power supply. The display can also be connected to the phone without additional power, but it just mirrors the phone's display (which works fine for me too).
The HDMI part works all the time (as stated in the original review).
The seller was also quick to respond to the original review, does have a warranty, and is willing to honor it. So it was extra stellar, but didn't have to go that route.
So after all, I'm happy with that.