@PeterP 

Have you contacted Spectrum? I've got Spectrum and before I can even use a modem that they provide as a tech exchanges it for the same model due to a malfunction they have to issue some commands to condition the modem to work. This is quite normal. See https://www.spectrum.net/support/internet/activating-new-modem-b?redirected=truehttps://www.spectrum.net/support/internet/activating-new-modem-b?redirected=true please.

Spectrum also had a list of preferred devices you can attach to the signal. Signal,

If you have 100Mbps service, that TP-Link Modem Router is approved for use, see https://www.spectrum.net/support/internet/compliant-modems-charter-network/, but you still have to provide Specturm with the MAC address to have it work.