Question 5Ghz cuts out after connected to modem

[Banks12345]

[Moderator Note: Moving post from Graphics Cards to Networking.]

This is probably the strangest internet problem I've ever encountered. I have a Deco XE75 router, my modem is the CM1150V, and my Cable internet provider is Cable America. I have had 5Ghz work on their previous modem/router combo. However, when I upgraded to the deco, and originally a Hitron modem I was unable to get a 5ghz connection. I tried troubleshooting and eventually did a RMA, 2nd one has the same issue. I then switched out the modem, for the main reason that the Hitron had the Puma chipset, which I was unaware of at the time of purchase. What possibly could be the issue with my 5ghz band? The band is fully visible as long as the internet is out, the moment there's internet my 5ghz drops off the wifi list. I have talked to TP-Link tech support quite a bit, and Cableamerica last week fixed a cable issue outside the house, as before there was a higher than average cable frequency iirc, like 70-80, I was told 50-60 is normal and that they were surprised i had internet at all, wish i remembered what the measurement unit was..