Modem/Router isn't connecting to ISP

To make a long story short, old modem stopped working, got a good deal on a used C6300 on eBay, and it won't connect to the internet.

My ISP is Shentel. 4 days ago I spent about 30 minutes while on the phone trying (and failing) to get it to connect. We made sure the cable connections were secure, MAC address was correct, factory reset the modem, still no connection. The lady said every time it would try to connect it would cut off and if it was used with a different ISP beforehand it would make the modem incompatible. I've never heard of that and our previous modem was a used Xfinity gateway and it worked fine for the time we had it so I don't think that's a problem.

I found out this modem has a button to turn on/off the Wi-Fi radios and they were turned off at the time of the initial call. Today I called back and this time I made sure they were on. Still did the exact same thing. To make sure this was a problem with the modem itself. I hooked up an Arris Surfboard modem and that connected very fast with no issues. I've looked through the manual and searched the internet for answers all day and I've come up with nothing.

Is my modem/router busted or am I missing something?