Very few people know that you can save yourself some money by buying a T-mobile router on Amazon. Downside is having to flash it to have the latest firmware free of vulnerabilities. In this guide I will show you how to flash a TM-AC1900 to a RT-AC68U then into merlin custom firmware. I have tested it myself and can confirm that it works.
Update: Asus began rolling back firmware on the stock setups. They are not providing Merlin with the source for Ai-Mesh so you need to be on stock to use it. I have updated the guide to provide instructions on how to prevent the rollback from happening. You can also use this guide to recover from a rollback.