Part of the Furniture
Verizon internet is not yet available to me, but I would switch if it works well and is available, I have no love for AT&T. They do have an option I can buy a static IP and setup some kind of cascaded router setup, but it's 300 dollars the first year and like 200 per year after that.
As far as IPv6 settings, I didn't have it on with the netgear. I only tried that as part of troubleshooting and left it on. It was having these issues without it on, or with it on. With the older netgear, I didn't really do anything on it, and with ATT it was only the setting for the dmzplus mode and I didn't change anything else, because it did work. The ATT router has very poor range, the netgear router at the time was to see if I could get better signal and I was able to. At that time only had one xbox so it wasn't really a problem. But like I said that router took a dump on me, it was about 8 years old.
As far as public IP address, it maybe isn't really public, it starts with 172. I'm trying to figure out where to see it with the ISP router, it's also a 172 but something else for the second number. I was googling how to see it, but it said if its 172 or 192 its probably not the public ip. I dont know. The asus router is 192.168.x.x locally on that network.
In any case, this weekend I'm going to just go ahead and factory reset everything, that should turn off ipv6, I think its off by default. I might pick up a used netgear router like the kind I had before off ebay, I saw them pretty cheap, maybe just switch back to that.
172.16.x.x through 172.31.x.x are private IPs. Merlin uPNP won't work with that, and if your ISP's WAN is 172.x.x also, you're basically stuck as even in DMZ, they are doing a NAT somewhere up the line. You don't have to worry about masking your IPs here since nobody could ever hack or reach those 172.x IPs. Unless their IP falls outside of the range 172.16.x.x through 172.31.x.x. The rest of that network is public.
In this case, getting IPv6 up and running may be your only option to avoid double (possibly even triple) NAT. Generally, for outbound connections, multiple layers of NAT is not an issue, so perhaps your only problem is that uPNP isn't functioning. If the ISP router doesn't let you set your own LAN subnet (sounds like it doesn't if it is extending the WAN subnet to the DMZ device) then running stock AsusWRT instead of merlin may solve your problems. If you look in your log on the Asus you'll probably see miniUPNPd messages saying you have a private IP and disabling itself.
But even if you got uPNP functioning on the Asus, and have DMZ enabled on the ISP router, if the ISP router WAN IP is 172.16 through 172.31 also, they are still doing a NAT somewhere upstream so uPNP won't work anyway. Or maybe the 172 you're seeing is just a management IP and their WAN IP is in fact public.
I'm wondering if maybe with the Netgear you had IPv6 up and running and just didn't realize it....