Oh no! You are confusing 5GHz WiFi with 5G cellular network! ROG3 supports both, YES, BOTH!
In dual-band wifi, if u turn it on, the device can connect to 2 WiFi networks at once, BUT, one has to 2.4GHz and the other HAS TO BE 5GHz. It CANNOT connect to two WiFi access points with same frequency! 2x 5GHz or 2x 2.4GHz - will NOT WORK!
HOW TO USE? -> Connect to a primary network first, ONLY THEN, turn DUAL BAND SWITCH ON and select a second WiFi INSIDE the dual band option. It will work, easy. 🙂
Fot 5G cellular (== SIM card) network, u may check with your cellular operator for that.