You'll need to ask the holder of the account the device is linked to release it. See, for example, https://forums.xfinity.com/t5/Your-Home-Network/Transfer-modem-to-new-account/m-p/3246363#M305318 and replies.
Hi there, Jplopez.
What kind of modem did you buy? Was it an Xfinity branded modem?
I wanted to reach out one more time before closing out your post.
Did you have any other questions?