Update on Sept. 16, 10:29 am: Apple has fixed the issue in an update. You can install the update on your iPhone device by opening the Settings app, then selecting General, and About. Your iPhone should automatically recognize a Carrier Settings update is available and prompt you to install it. For those who took T-Mobile's advice and didn't install iOS 10, you can now proceed. T-Mobile also published a support document detailing how to install the update.
Thursday afternoon T-Mobile's Twitter account posted a strong warning: iPhone 6, iPhone 6 Plus, and iPhone SE users "Do not download iOS 10".
According to the tweet, the update is to blame for connectivity issues between those iPhone models and T-Mobile's network.
Apple is aware of the issue and will have it fixed in the next 24 to 48 hours, according to a second post on T-Mobile's Twitter account.
In the mean time, those who've already installed iOS 10 and are seeing issues, T-Mobile claims a simple restart of your iPhone will restore connectivity.
You can power-cycle your iPhone by holding in the power and home button at the same time until you see an Apple logo displayed on the screen.
Apple's release of iOS 10 hasn't been perfect. During its first hour of availability on Tuesday, iOS users reported issues with the update stalling just as it finished. Those impacted by the issue were required to use iTunes on a computer to reinstall the update.
We have contacted T-Mobile, asking for further information about the issue and the impending fix. We will update this post when we know more.