The T-Mo Report (who knew there was a site dedicated to T-Mobile news?):
According to internal documents shared with us here at The T-Mo
Report, the blocking of Apple’s privacy-focused service is
actually due to a conflict with existing content filtering
services on T-Mobile. […]
This seems to indicate that the blocking isn’t actually
intentional by the carrier, but merely a necessary step to ensure
their own services work properly. The blocking affects very few
customers in practice, and it seems that there are currently no
plans to expand the blocking of Apple’s service to standard
Impacted customers will receive one of two error messages, shown
below (sourced from the same internal document), stating that
their plan isn’t compatible with the iCloud Private Relay service.
As I wrote in
Another thing to check: in Settings → Cellular → Cellular Data Options, make sure “Limit IP Address Tracking” is turned on.