Have just been doing some reading on the T-Mobile forums and it looks like this is most definitely due to T-Mobiles implementation of CG-NAT, apparently they do not offer Static/Public IP’s unless you are a business user.
Users of Starlink have a similar issue: Starlink (ISP) with ARC: CG-NAT Requires VPN Workaround (See Staff Post) - #18 by ipeverywhere