XG Home in bridge or gateway mod?

Hello,

I’d like to add a Sophos XG home firewall to the following configuration:

WAN -> Cable Router (Bridge Mode) -> Router -> LAN

The main router is a FritzBox running LAN, WLan, wired phones and DECT.

Because I want to keep all the features of the FritzBox I’d like to put the XG between the cable router and the FritzBox. As the cable router is in bridge mode, the FritzBox gets its WAN-IP with DHCP direct from the provider.

WAN -> Cable Router (Bridge Mode) -> XG -> Router -> LAN

 Should I configure the XG in gateway or bridge mode?

Thanks for helping me.