Devices through UCG-Fiber getting mis-geolocated as Poland

I have a UCG-Fiber gateway with a Hitron Coda56 modem. For wifi I use a U7-Lite. These are physically in New Jersey, using Xfinity cable internet.

Recently Google's geolocation service been wrongly reporting that "Warsaw, Poland" when I'm connecting through the UCG-Fiber. I've changed the IP address by temporarily altering the MAC address of the UCG-Fiber, but Google still thinks my wired-only PC and wireless laptop and phones are located in Poland.

I tried connecting my PC directly to the Modem (bypassing the UCG-Fiber). The PC received a new IP address, different from the UCG-Fiber, and the PC correctly geolocated as being in New Jersey. I'm verifying this by calling navigator.geolocation.getCurrentPosition() from chrome devtools (also same result in edge and firefox).

As soon as I reconnect the PC via the UCG-Fiber, the PC starts getting geolocated as Poland again. If I disable my ethernet adapter on the PC entirely, then geolocate, it immediately correctly identifies my PC as being in New Jersey (I have this set in Windows settings). If I turn the wifi off on the phones and use cellular data, they correctly identifies as being in New Jersey.

I don't use any VPN on most devices, except for one phone where I occasionally use AdGuard VPN. I had been using Cloudflare DNS on the UCG-Fiber. I've swapped it to 8.8.8.8 to see if that helps, but it doesn't appear to have made any difference.

I am truly at a loss here. Every time I open Google maps, or look at Google shopping search results, it displays Polish locations and prices.

I've already reported this to Google's IP address geolocation reporting portal, but even a new Xfinity IP address has the same exact problem.

Can anyone suggest what to try next? Is this somehow related to a feature on the UCG-Fiber that I can change?