macbook pro 2017 and dell U2417H hdmi adapter issue
I have same problem with MBP 2017 15" with plain Full HD monitor.
MacOS always says it is connected but no signal to the monitor.

As Jackson1442 says, rebooting sometime works but not all the time.

My MBP 2014 has no problem so I doubted my hub and then the hdmi cable. I replaced those but no changes.

I now doubt about 2017 model itself as Mohannad Kattan mentioned. It is so annoying.

I will try USB-C to HDMI cable without hub.