MR6500 Port forwarding Not working

Despite what it shows in the MR6500 WebUI, neither Port Forwarding nor Upnp opens the port of the device inside the private network. I can access the device's port (in my case 31500 for the HTTP service) from within the private network (10.0.X.X:31500) with no problem. But when I'm trying to reach the device from the WAN, let's say 107.204.X.X: 31500) it times out and refuses to connect.

I connected the laptop to the MR6 via WiFi and "canyouseeme" reports the port closed. Same thing when attaching the laptop to the MR6 via an ethernet cable.

I found an AT&T article that says that the MR6 Port Forwarding does work because of limitations in the software. To use UPnP instead. But it doesn't work either. By the way, I checked the device using another ISP connected via a LAN router with the port forwarded and I have no problem accessing the device

Any idea on how I can use this pricey brick?

Thanks.