What is the Ethernet cable from your adapter connecting to?
Give us all the equipment between then end of the Ethernet cable and the Spectrum coax cable. Modems, routers, switches, etc…
Note: if you are disconnecting your WiFi access device and plugging directly into the cable modem, as in only a modem, and not a router, that could be a problem.
Some ISPs lock onto the first devices MAC address (Media Access Control address, which looks like this nn:nn:nn:nn:nn:nn) it sees, and only the ISP can reset it.
Some ISPs allow power cycling the modem to latch onto a dew MAC address.
Then again, if you are plugging into a LAN Ethernet port on your router, that should work, and your router is connected to the cable modem.
Many ISPs provide a combination cable modem, router and WiFi device. It should have some LAN Ethernet ports.
And if you have a separate Ethernet switch, we want to know that as well.