Switching from customer provided modem to Xfinity Gateway

I received an email stating that my cable modem is no longer receiving security updates and needs to be replaced. I have Xfinity Voice, so need a cable modem that supports that. None of the supported options listed by Xfinity are available at retail anymore, so I would like to replace my old device with an Xfinity Gateway. However, all my attempts at doing so online have resulted in errors. Any help would be greatly appreciated!