Yeah, I'm using a USB-C to DisplayPort cable with the Samsung. What I didn't say was that I've disconnected my Dell for now (it was in portrait mode) and I'm just using the Samsun 4K 27" until I can figure out something that works better.
Maybe there's something up with the cable. Have you tried another one? Also try another port. The electrical characteristics sometime seem to be very sensitive for DP.