Panoramic WiFi Modem LAN port problems

Theoretical connection I'm paying for: 300 mb/s

I have the DAR1682, according to the "My Equipment" page, which is the Arris TG1682G modem/router/WiFi combo. The WiFi works fine, no problems.

The four LAN ports do not work well at all. I have tried several different ethernet cables, old Cat 5, Cat 5e, and Cat 6. All of which only have throughput of about 2mb/s maximum and ZERO minimum.

I have already returned another modem and was given my current unit. This one worked initially, but has since failed.

Again, the WiFi works fine, as expected. But I prefer my connection to be via ethernet. Some pieces of equipment are only capable of LAN connections and NOT WiFi.

Has anyone else experienced this problem? Is this the best that Cox has to offer for a modem? I do not need an all-in-one modem/router/WiFi unit. Is anything else available, or do I need to go back to my SB6183?