Use my own modem

Hello, I used my own modem in the past.  It allowed me to run in bridge mode so I could use a software defined VPN (TpLink Omada) to connect my home and office sites.  I recently upgraded my xfinisty speed at home and received a new gateway.  If I want to go back to using my old modem, will the xfinity network still give it an IP address and allow traffic to pass?  How long does that handshaking process take?