Controversy abounded when reports showed that T-Mobile was blocking Apple's new iCloud Private Relay feature on its network. However, T-Mobile has put those claims to bed by asserting that it's not blocking anything.
After reports circulated all over the internet, T-Mobile was forced to step in and explain that the only time Private Relay was blocked was when users had parental controls enabled on their devices.
In a statement to 9To5Mac, T-Mobile said, "Customers who chose plans and features with content filtering (e.g. parent controls) do not have access to the iCloud Private Relay to allow these services to work as designed. All other customers have no restrictions."
However, 9To5Mac claims it doesn't have any of those content filtering features enabled, and it's still unable to use Apple's feature as intended. Hence, it's still relatively unclear what is happening. Something is preventing them from using it, but if T-Mobile's statement is accurate, it doesn't seem that the carrier is responsible.
The original error message on iPhone is as follows:
Your cellular plan doesn't support iCloud Private Relay. With Private Relay turned off, this network can monitor your internet activity, and your IP address is not hidden from known trackers or websites.
That certainly sounds like something on the carrier's end, but it could be some other kind of error that's causing the problem.