Unregister Modem from Deactivated Account

Hello

I am trying to sell a modem + router that I bought myself (not through comcast) and have used for about a year. I deactivated my comcast account as I no longer needed it and am now trying to sell this, but I am wondering if I should have deactivated/disconnected the device from my account.

I can no longer access any information as I have disconnected the service, nor can I talk to a support agent as my phone number is no longer associated with an active account. 

Is there a way to verify that I can safely sell this to another person? Thank you