Need to set up a Sophos UTM based firewall behind my Comcast DPC3941b modem, would appreciate help.

I have a DPC3941b modem that has a static IP assigned by comcast. I've built out a Sophos SG 115 with UTM 9.3 and want to set up a site to site VPN tunnel between said site and my main office. I've dont it before at other sites, but all the other sites have been fiber and i never had to deal with a cable modem.

So my question is,how do Iproperly setup the Comcast Modem so the External IP will not be NAT'd so as to allow the UTM to recieve the external IP address of assigned to said comcast modem? Also, I need to make sure the comcast modem isn't trying to assign out IP address and what not.

Essentially I just want the comcast modem to pass through everything to the Sophos Firewall and let the firewall run the show.

Anyone know the settings I need to configure within the Comcast Modem?