https://www.theverge.com/circuitbreaker/2017/9/15/16316224/apple-watch-series-3-tmobile-data-throttling-512kbps

T-Mobile will limit data speeds on the Apple Watch Series 3 to a maximum of 512kbps despite the smartwatch’s support for LTE connectivity, a spokesperson has confirmed to The Verge. As MacRumors noticed earlier today, the throttled speeds are mentioned on the carrier’s website. Apple isn’t yet advising customers of T-Mobile’s capped data performance.

“Customers can do everything they want to do with the watch at 512kbps,” a T-Mobile spokesperson said by email. “If customers feel they need high-speed data, they can choose high-speed data with paired DIGITS for $20 with auto-pay.” If auto-pay is not enabled for your plan, the cost is $25.