I would call Apple Support and have them sort out the problem.
One question though. Where did you get the USB-C to HDMI adapters from?
I recently bought an iMac 27, 3,4 GHz core i5, Retina 5k, late 2017, 8+8 Go ram memory, running on High Sierra 10.13.2. It is connected to two Acer G247HYL external displays through USB-C/HDMI connectors. When I turn on the iMac, the displays do not wake up. They don't appear in Preferences system/Monitors even if I click on Option to detect monitors. The only way to wake them up is by unplugging and replugging the HDMI connector, which is inconvenient. I have reseted the NVRAM (start in Option + Command + R + M), reinstalled the system, double check the connectors (which are brand new) and the displays (which worked well with my previous iMac 2011). Interestingly, the external displays wake up automatically when I restart the iMac. Any suggestion would be much appreciated. Thanks.