FIX: T-Mobile Wireless (WiFi, not 3G) CellSpot / ASUS TM-AC1900 Port Forwarding Not Working

Problem

You try to forward ports on your new $25 deposit ASUS Wifi awesome-router and it doesn't work. Neither does UPNP. Or maybe it works but not for very long.

What's happening

Those clever folks at ASUS added a NAT acceleration capability. Unfortunately, as best I can tell, it basically does nothing much better than crash regularly. This is why you may have noticed that forwarding works occasionally after reboot. I, personally, found this to be very rare. I've also found that the NAT acceleration isn't noticed when it's disabled. I don't have Google internet, but I'm pulling 80/20 and the router is doing fine.

Fix

Go to LAN -> Switch Settings. It will probably hang. Worry not, your router probably just crashed. You likely lost internet. Give it a full minute and refresh.
Go to LAN -> Switch Settings. Set any setting that has the word Acceleration (or NAT) in it to Disabled. Your router will reboot and your ports will be forwarded.

Update: Around December, for no apparent reason, my NAT rules all stopped working again. Nothing had changed on the router (well, as far as I know ... we rarely need that functionality). See the comments for what I did to repair, but I'll summarize: Factory Reset and some other steps.
I couldn't find better information to help with this, so I went nuclear and blew away the entire configuration. It worked, but ... yuck!