Connected but have message "Can't connect to your device"

I am connected but my account has a message "Can't connect to your device". I replaced an old modem with a new one and made sure that the new one was on the "compatible" list. My old modem, even though disconnected and turned off, is still showing in my internet devices - along with the new modem. I was unable to remove the old modem from my account devices and spoke with someone in the Technical Dept who told me they would have it removed. That was a month ago and the old modem is still there. Is this why I am receiving the "can't connect to your device" message and is this affecting my internet access, speed and/or ability to receive updates?