Have Internet access, but can't connect with many sites on G34

I’m a Cox Communications customer in Scottsdale, Arizona. About 2 weeks ago I began to notice a gradual slowdown in my internet speeds. Videos were beginning to buffer a lot and frequently visited websites were downloading slower and slower. Reps at Cox told me that my Arris DG2460 modem was outdated and that I should get a new modem.

Based on several recommendations, I bought the Arris Surfboard G34 and set it up this morning. It’s a Cox approved modem compatible with their system/equipment.

When I’m using a modem I’m nearly always on my Dell Optiplex 7020 desktop. I connect it to my Arris modem on Wifi. My wife uses an iPad (also Wifi). Occasionally, I’ll check the appearance of a website on my Android phone. Additionally, we stream video and TV through our Roku device.

So, we use 4 different devices with the modem, but never simultaneously. After I set-up the new G34, the first device I checked for connectivity was the desktop and it seemed fine. Eventually, I checked and set-up the other devices, Initially, it seemed the changeover was a success and we were looking forward to better, faster online experiences.

I went back on the desktop and suddenly discovered I actually could not connect with many of the websites I frequently visit or use. I was getting the error message “ERR_ADDRESS_UNREACHABLE”

The post feed on Twitter would not download. YouTube videos played without buffering, but audio had some cracking and soft pops on mostly all music. I called Cox and their first response was that I should have bought a Panoramic modem. 

I checked the wife’s iPad and found I could visit all the websites I couldn’t see on my desktop, plus everything was faster with the new modem. I had the same positive experience with my android phone… it was much faster and smoother on Wifi. Our Roku streaming continues to deliver very well. So, everything works better and faster with the new modem except my desktop… which is actually worse than before.

Does this mean I suddenly developed a problem with my desktop that I didn’t have before turning on the new modem? I’m NOT a strong tech guy, but this seems strange. I’m baffled by what I should do or try next.