Wi-Fi No Longer Working After Hardwiring Desktop

[Taekwonty11]

Hello, I have a 1 GB internet plan. I am using a  Netgear Nighthawk Cable Modem, model CM1200. For a router, I am using an ASUS RT-AX56U. 

Basically, wi-fi was working fine until I plugged an ethernet cable from my modem into my desktop computer. The moment I did so, wi-fi stopped working. The desktop computer connected to the modem work fine internet-wise. 

The router has a red light where it should be white, signifying an internet connection. Going to router.asus.com comes back with the error, "Your ISP's DHCP does not function properly."

I have tried unplugging/plugging back in and MAC Clone, all to no avail. 

Happy to answer further questions to get to the bottom of this... 

[EG · Accepted solution]

Only one port is active on the CM1200. You can't use both without getting an additional public / WAN IP address from Comcast. Plug the desktop into your Asus router instead.

[Taekwonty11]

Thank you for that information, I was not aware of that. Here is my issue now:

When I plug the router, and only the router, into where hardwired connection was working, I am getting the same error. I even tried factory reset on router and setting-up a new network to no avail.

When I unplug router and plug hardwired back in, boom, fast internet I am using to reply to you now.  Why would it be simply plugging router back in will not work?

[EG]

After connecting the router to the modem (router powered down), you have to power-cycle / power-down the modem first. Wait 30 seconds and plug it back in. When it fully comes back up, power on the router. 

As an FYI, you need to power cycle/re-boot the modem each and every time that you change the device that is being directly connected to it in order for that device to be able to get an internet connection.

[Taekwonty11]

EG, thank you so much for your help here. I am now, it appears, back up and running with 400+ MBPS wi-fi speeds on laptop and 850-920 MBPS speeds with desktop hardwired to router.

Really appreciate you taking the time!

[EG]

My pleasure ! Happy surfing !