When LAN connects to desktop, WiFi dies

We are having a frustrating problem with our WiFi and I am hoping to get some guidance.  The WiFi works beautifully as long as the modem is not connected to our Dell desktop (Windows 10).  When the modem (Asus AC1900) is connected to this particular desktop, the WiFi shuts down completely.  Of note, this set up worked perfectly for 6 months before this problem started with no obvious changes to the set up.

We have connected to another PC laptop and a Microsoft Surface Pro and these devices do not have the same effect; the WiFi continues to work fine with normal speeds when these are connected, suggesting that there is something in this particular Dell desktop screwing things up.  

We have spent countless hours on the phone with our ISP (Jaguar Communications - Fiber Optic), Asus customer service, and even our Sonos guy, but have come up empty.  Googling the problem and posting to other forums has not lead to a solution, either.

I have tried to fix the problem through BIOS, but there is not "network" section that I can find in my BIOS.

I was suspicious for some malware or virus on this desktop, but I've scanned it multiple times with updated scanners and not found anything.  All my drivers and firmware are up to date.

So what could be going on with this Dell desktop that causes the WiFi to fail when the router is connected to the desktop?

Please let me know if you need more information that might provide more clues, and I greatly appreciate any insight!

Thanks,

David