I have a house that is located on top of a mountain where there is no cable service and has a smaller cellular bandwidth as my ISP.

So far, ever since I installed the AX21, the phone app can no longer communicate with the AmCrest Security Cam DVR when I am not at the house,  no matter how I reconfigure it.

Here's the setup:

WAN <--> NetGearNighthawkHotspot cellular device via AT&T 

The above NetGear device is set to an IP passthrough so it does not have WiFi, just a dumb device to get onto the cell network and Internet.

Behind that NetGear device is the AX21.  So, 
WAN <--> NetGearNighthawkHotspot <--> AX21 <--> AmCrest Security Cam DVR
 

I have even reserved a  static IP for the AmCrest Security Cam DVR inside the AX21 firmware config.

The AmCrest DVR uses Upnp and P2P for the phone app to get in. Previously, when the Netgear device was set up to serve WiFi and act as a router without the AX21 router, the DVR app worked fine. But, once the AX21 Router is in place, it cannot get through anymore.

Of course, when I am on premise using the local WiFi provided by the AX21, it worked fine. When I turned OFF the WiFi to go through from the outside, it cannot connect. Very rarely, I can get in on the App but that is more like 1% of the time.

Any idea on how I can resolve this?