I'm wired directly from modem to PC - same issue, web browser redirects me to Cox's page that says my device is not activated, and hitting the activate button gives me the 403 error saying the server understood the request but refused to authorize it. Can't access any other webpage, just get redirected.
For troubleshooting, there is a reboot, reset and reprovision. You can reboot (unplug) and reset from your Cox account. A reprovision may fix your problem; however, only Cox can reprovide your account. It's deleting your privilege for Internet service and then recreating it.
I recommend sending an email to firstname.lastname@example.org. Just briefly explain what you've done to no avail so perhaps a reprovision. They can look into your modem, signal levels, your bill is paid, etc.
I think you need to include your name, address and perhaps phone number in your email. You could do this over the phone but the holding stinks and you don't know who or where you'd be dealing.
Chat is a nice feature but I think it's just a bot.