strange Foscam camera issues with R6300 and R6900

TL;DR: foscam cameras work great on other routers outside the wifi network, but do not work reliably with two Netgear routers now. On the latest R6900 router, cameras work well on Cricket (AT&T) network, but do not work well on Verizon LTE network. Same setup with ASUS / Linksys worked great for years, why?

Story: 

We have several Foscam cameras that were set up and worked great on my old Linksys WRT54GL (running Tomato) for years.

Then we switched to Cox Gigablast (fiber optic network), and Cox gave us a brand new R6300 AC router in January of 2016.  I set up the static IP addresses of the cameras (outside the DHCP address range), set up port forwarding for each, just like how they worked on my Linksys WRT54GL.

The cameras would come great inside my network, on my Galaxy Note 4, iPhone 6, and other android / iOS devices.  As soon as I switched off wifi, and tried to use Sprint LTE or Verizon LTE, the cameras would be very difficult to access.  They may come up initially for a few seconds, and then the data stream would just disconnect and I woudln't be able to connect to the cameras again.

Thinking that Cox network was the issue, I debugged with Cox and didn't get a resolution.  I then tried putting my old WRT54GL on the Cox gigablast and the cameras instantly came on outside my network, voila!!

So I shelved the Netgear R6300 (running the latest firmware at the time), and switched to an ASUS RT-N66U router, and it worked awesome from January til 2 weeks ago, when the ASUS router lost all wifi connection and refused to connect to wifi clients despite multiple resets.  So I had to come up with a new game plan.

I went to CostCo and purchased the Netgear R6900 (AC1900) router for $159, and set up the static IP addresses of the cameras, and the port forwarding as previously.  I put the Netgear R6900 on the network, and everything worked great inside the wifi network.

Once I turned off wifi, and switched to Cricket 4G or LTE (I switched from Sprint to Cricket), the cameras would come up, a bit slower than the ASUS N router, but they would come up reliably and the data stream would work.   I then tried with my Verizon LTE iPhone 6, and the same problem occurred - the camera stream would come up for a few seconds, then the data stream would die out and I would be unable to connect to the cameras.

So a VERY ODD problem!! The problem is I can't reliably access my Foscam cameras outside my network, very sporadic initial access on my Verizon iPhone 6 on LTE (and this exact set up worked perfecly with the ASUS router before it died), and it works reliably with the Cricket / AT&T network.

I'm thinking the problem has to be with the Netgear routers:

• Same Foscam camers
• same Cox giglablast network
• same client devices (Android / iPhones, all same apps)
• The ONLY difference is now I'm using a Netgear router, as opposed to the Linksys WRT54GL or the ASUS Rt-N66u

FWIW I'm using the latest firmware with the Netgear R6900 - V1.0.0.4_1.0.10.   

I'm kind of at a dead end trying to figure out what the issue is with Netgear.

Any thoughts from the advanced tech heads here?