5 GHz Wi-Fi on AC750 Archer C20 was not working. I could not connect to the 5G WiFi on my iPhone or my PC. I was unable to find a solution online.
The settings that were not working were the default ones.
Connect to your router. Usually 192.168.0.1. Navigate to Wireless 5GHz and change Channel to 40 and Channel width to 40MHz.
This solution worked for me. Experiment with other Channels and Widths in case you have this problem.