Arris TG3482G and bridge mode: DHCP fails

I want to use my own router (Apple Airport Express) and don't want to double NAT so bridge mode is preferred. 

When I set up the  Arris TG3482G (Xfinity Xfi xf6) in bridge mode, the DHCP address from Comcast's network (with the public IP address) does not get through to the Apple router.  I can see in the configuration of the xf6 that it has an IP address (why? Isn't it in bridge mode?) and that all other services are (correctly) disabled 

What am I missing? Setting up the Apple router in DHCP mode, and connecting it to the first port of the Xfi, rebooting both, one should have a bridged mode network. But the Apple router never gets a DHCP assignment, it just has the link-local, self-assigned zeroconf IP.