If you are using an older desktop computer you may find that it doesn't have a Wi-Fi card install and you will need to connect an Ethernet cable from your Archer AC1200 modem/router to your desktop.
The Ethernet cable will most likely be a bright yellow or blue. Connect to the yellow LAN port on your Archer AC1200 modem/router and then you should find the same kind of port on the desktop to connect the other end of the cable to.
Sounds like there might be a setting wrong with your desktop Network card. I recommend if you haven't already done so, to restart your desktop and modem and retest.
I have also requested for one of our Helpdesk staff to give you a callback to continue the troubleshooting, and they will give you a callback later on today.