Netflix no longer streaming

I gave the questions some thought and have a couple for you both to consider if you have not done so already.

Do you use the gateway with a third party router on your network or just the T-Mobile gateway? 

Do you have dual stack enabled on the streaming devices you are attempting to use?

The reason I ask is that Netflix is among the largest domains using IPv6 for streaming content delivery and IPv6 does provide some benefits over IPv4 for content delivery. Using dual stack IPv4/IPv6 on the devices would probably be best. With IPv6 on the clients it would also circumvent the 464XLAT overhead due to not requiring the CLAT / PLAT translation between IPv4 & IPv6. I suggest to check the clients and make sure they do run dual stack. If there are any software/firmware updates available for the TV or Firestick also apply updates.

I use Netflix on multiple types of clients here including a Firestick and have not experienced issues but I also do not use an additional router on my network. I have kept it simple and direct. If you do run an additional router with your clients on a network other than the T-Mobile gateway segment it would be worth running the same client(s) on the T-Mobile gateway network to confirm operation as such. 

Just a couple of thoughts as the problem might still be upstream and not a local network issue. If you have not investigated the DNS servers your clients are directed to then I would also suggest to experiment with the target DNS servers for the clients. When I have experienced some internet related communication issues here my alteration of the DNS source away from the gateway did help so I tend to still avoid allowing the 192.168.12.1 gateway to be used for DNS resolution.

Good luck with your endeavor.