all pdn ip connection failure

I think it is important to keep in mind that T-Mobile is a mobile carrier first and foremost. It is common for cellular carriers to use IPv6 due to the sheer volume of required IP addresses. Sure the IPv4 to IPv6 translation does impose some limitations but seriously if software vendors would embrace IPv6 the performance for communications would probably improve and the limitations imposed by IPv4 and port forwarding that requires NAT could be remediated. IPv4 was adopted for use in 1980. Seriously the problem is more with software vendors that use the old IPv4 protocol stack and do not leverage IPv6 technology. 

I will not try to defend T-Mobile and their practice of over subscription in some market spaces. If the towers and backhaul paths are over subscribed and excessive traffic prioritization is required well that is indeed an infrastructure issue that needs improvement and well that takes more money and engineering. My experience here with the T-Mobile solution has been very positive most of the time since January of 2021. When work was done to upgrade from n71 to n41 it was indeed a week of disruption. Over all most of the time it works. It really depends upon the local environment. We are in a rural location not dense urban so the saturation is not bad. Will that change? Time will tell as the area grows that could well be the case if they do not have proper planning. So far it has been really good. Is it perfect? No but it hands down beats the life out of the DSL solution we had in CA for reliability and consistency.

There are lots of variables in the cellular signal delivery so the problems there may be a combination of factors and NOT just a IPv4/IPv6/IPv4 translation complication. The fact that there are three different gateways and odd issues with each does not help. OK so four if you include the LTE gateways that are still in service. The software and/or hardware issues with the gateways are just as likely to be a factor. Each edge access gateway solution has its unique challenges so I just don't tend to believe there is any single reason. I find it helpful once every 45-60 days to just reboot the gateway to flush it out. That isn't always necessary but it seems to clear out odd behavior. Sometimes it might seem like a network issue only to be a problem with the local client. My iPad Pro is notorious for needing a reboot whereas my MacBook Pro or my Linux clients tend to have few issues. The Windows client still here well it usually chokes on Microsoft updates and it is a Alienware gaming rig with plenty of resources. I see Linux looming on the near future for that client. There are just too many variables to blame the CGNAT alone. There is nothing else for us here that can touch the cost and/or performance of the T-Mobile internet solution. We just are in a great location to get good cellular internet communication.