To tell if the modem is bridged or not, look at the routers web page, Status/Device Info/Wan Section, if there is a 192.168.0.# address in the WAN IP address field, then the modem is not bridged.
I presume PF may or may not be needed. I managed to help my friend get his XB1 NAT open on his router after about 2 hours of trying to figure out why it wouldn't connect correctly via LAN cable. Wireless it would however plugging in the LAN cable, Moderate NAT was seen. I looked at the connection status on his router (DIR-868L) and saw that the wireless was still connected when the LAN cable was connected. Could not figure out why wireless was connected still. Did everything we could thing off. Eventually told him to contact XBL support and ask whey the wireless on the XB1 was still connected when LAN cable was connected and preferred. Haven't heard back yet.
Just thought I'd share this experience should anyone see this with there XB1. We presume that when the LAN cable is used, the WiFi should not be connected or enabled. We believe this is the cause of Moderate NAT on XB1. I'll post back with results.
As for Greenhawk, we need to make sure that the ISP modem isn't doing any NAT'g and that the NAT is solely on the 5500. PF shouldn't bee needed on this model router as the SB system and uPnP should take care of the port configuration. I've had not needed to do this for my 2 xboxes. Just make sure they are connected and give a higher priority if you need if other devices are online at the same time.
Be sure to network factory reset the xbox as we've seen for some reason the xboxes are not fully clearing out the prior network settings and somehow gets hung up and causes Moderate NAT. Usually once the xbox is reset, Open NAT is seen.
I called Cox Cable and they want to send me a new Docsis 3 modem. I talked to the rep and he told me that I currently do not have a router in my Arris. We do have our land line phone service through the modem. I informed him that I have my own router.
Do you know what I have to do in the DGL-5500 to loosen the NAT?