T-Mobile Users Can't Connect

Given that your issues is with T-Mobile users only and not with users on other networks the problem is (most probably) somewhere in the T-Mobile network. You'll need to contact them and see if they are blocking your IP address for some reason. Note that they might not just be blocking your IP address - they may have decided to block (say) 3.213.214.0/24 and you're just a side-effect of that.

I know it's disruptive but you could try using a different Elastic IP address which is not in the same range and see if that helps. Probably worthwhile setting up a test system on a different Elastic IP and see if the T-Mobile users have an issue with that.

I have seen this before when a VPC is configured with the same CIDR range as someones public range and the traffic will never return because of a asymetric routing issue. Some have mistaken and setup 172.X.X.X/8 as a 255.0.0.0 previously.

Please can you confirm your VPC CIDR Range?