This is a cheatsheet on unlocking the HTC M9 based on two XDA forum posts. stifliz SIM Unlock post and this thread for T-Mobile users on changing the SKU. This method still requires a traditional unlock code (but it gets around the T-Mobile unlock tool method).

Overview of the Steps

• Bootloader Unlock
• TWRP Install
• SU
• Traditional Unlock Code

Unlock Bootloader & Root Phone

I followed this YouTube video to root the phone.

Change the SKU

Change the SKU in the /system/customize/ACC/default.xml file from 11 (for TMobile) to 0. There's a complete list of SKU's here.

Use ES File Explorer and enable Root editing to modify the file.

Reboot and Enter Unlock Code

Reboot the phone and then enter your unlock code.

Data, WiFi Calling & MMS Phone Settings

I had to create an APN setting for AT&T to get the data and MMS to work. There are AT&T instructions on how to create the APN settings for Android.

I also had to disable the TMobile WiFi Calling feature.