XR450 Port Forwarding not working

Hoping someone can help because I'm at my wits end trying to get my home security system working with the Nighthawk XR450.  I have an Amcrest 8 camera DVR system setup that has worked just fine with port forwarding on 2 previous routers, connected directly to the router via LAN cable.  For some reason I can't get the XR450 to transmit the video signal to the Amcrest app on my phone when I'm not connected to wifi.  To be clear, it works fine when connected to the wifi inside the house.  App on the phone works fine at home.  But as soon as I leave home and am off wifi and using mobile data connection, I can no longer see the video feed, although I can login to the DVR and see the router settings and can change them.  So it's for sure connecting, it just doesn't want to transmit the video outbound.  Reading some other articles with other routers, and it seems it may be a firewall setting.  I came across a suggestion to look at the WAN settings. Have tried playing around with DMZ, NAT filtering, enabling internet pings.  None of this seems to work.  Has anyone had any similar issues getting their security system to work with the XR450?