From DD-WRT Wiki

[edit] Hardware Specifications

[edit] Introduction

The Asus M-AC1900 (T-Mobile) is a hobbled RT-AC68U that can be upgraded to a fully functional RT-AC68U.
NOTE: This wiki is in the process of major editing due to broken links and outdated information.
SEE Flashing TM-AC1900 to RT-AC68U/P for more information.

[edit] Installation

[edit] CFE Downgrade is necessary to enable flashing firmware

This upgrade is a little trickier than most because the procedure involves not only navigating DD-WRT firmware, but navigating the CFE The newsgroups document that CFEs are able to control / deny firmware upgrades, which is the reason for 'downgrading' to firmware that does not prevent firmware changes.

******Updated guide to CFE Mod ****** here but doesn't mention flashing firmware

Version 2827 is firmware locked to only TMobile branded firmwares. Not only is the firmware locked (disabled Telnet), it installs CFE 2.1.2.2, which locks the router from any non TMobile branded firmware via recovery mode and GUI mode. The only way to change firmwares is to downgrade to an older TMobile firmware (1703 for example), enable Telnet, and then replace the CFE (preferrably with an Asus RT-AC68U version - such as 1.0.2.0).

Many details in this guide including links to the older 1703 firmware and all CFE's and utilities needed

if you're on CFE 2.1.2.1, which is not locked, and try to use the latest Asus branded firmwares, it will update your CFE and, lockout out all non TMobile firmware's again. Once you get to DD-WRT, doesn't matter anymore unless you decide to go (back) to Asus stock RT-AC68U firmware, latest copies.

For more information see UKEE's post here.

[edit] Links: verified firmware

There are numerous choices each with its strengths. DD-WRT / Kong are filled with features. Merlin to stay as close to the original firmware to ensure maximum performance.

• DD-WRT builds: FIRST flash a .trx build from the asus ac68u folder, to upgrade use the .bin file. Beta downloads

• Merlin and Tomato Builds: can be found in the tmo2ac68u package (broken link)

Merlin Changelog documents newest updates and firmware candidates

[edit] Links to tools and other files

Asus tools to revert FW ("Engineer" or UKEE to verify this). (broken link)

[edit] Links: verified CFEs

Verified: 1.0.2.0 from the tmo2ac68u package. (broken link)

CFE_ASUS_RT-AC68U_1.0.2.0 (US).txt

CFE_ASUS_RT-AC68U_2.1.2.1 (US).txt (??? user to verify)

[edit] Procedure

Before starting the process, obtain the tmo2ac68u package Broken Link. Tools and Firmware used in this process must be obtained and unzipped. Copy the mtdwrite utility and the target CFE to a flash drive. Fortunately, there is a (lengthy) YouTube video that demonstrates the different processes below. gatorback: because it is lengthy I skimmed through it ONLY when I was stuck. After you have installed \ downgraded T-Mobile's software, you will want to change the firmware: there is a good article discussing the various versions of firmware here.

    High Level  Steps:
      * Gain Telnet Access: flash TMO 1703 firmware via CFE webserver
      * Copy original CFE to USB memory stick
      * Copy data from original CFE to target CFE
      * MTD-write target CFE to boot sector
      * mtd-erase2 nvram 
      * nvram get bl_version
      * Flash final target firmware via CFE Webserver

1) Enable Telnet: Place router in recovery mode and upload TMobile firmware xxx.1703 (found in tmo2ac68u package Broken Link) through the CFE web UI. Enable telnet by navigating: Administration -> System -> Enable Telnet=Yes -> click 'Apply'

Alternative method: An alternative to flashing 1703 to enable Telnet
 **CAUTION**:  If you enable telnet, then erasing the NVRAM may disable efforts that enable Telnet.

Always perform a NVRAM reset after flashing firmware, reboot and allow 5 minutes to rebuild NVRAM variables.

    * HARDWARE NVRAM RESET:
    ** Power off the router
    ** Hold the WPS Button
    ** Power on the router and keep the WPS pressed for 10-15 seconds
    ** Reboot as necessary 

2) Check your CFE Version via telnet and change the CFE to 1.0.2.0. PRINT THE INSTRUCTIONS FROM THS LINK AND CHECK OFF EACH STEP PERFORMED:

    nvram get bl_version

This process involves telnetting and copying the existing CFE to retrieve data to USB stick, paste this data into the target downgrade CFE file, and writing the modified target CFE file back to the router.

3) Place router in recovery mode and install Merlin firmware via through the CFE Webserver recovery mode (easier).

4) Perform NVRAM reset

5) Flash DD-WRT and perform NVRAM reset

6) Go to your fridge and reward yourself with your favorite beverage and add your username here:

gatorback

[edit] Tests to perform after installing DD-WRT firmware

• NVRAM available immediately after install
• Speed Test measurements in 2.4 and 5 Ghz bands
• Verify USB 3.0 speed measurement
• Please update any other meaningful or useful test metrics / measurements

[edit] Further Information, References, Additional Reading

• GUIDE to CFE flashing (original)
• GUIDE to CFE flashing 2.0: (updated, but deprecated)
• tmo2ac68u tutorial - Flashing CFE - Firmware - Restore Mode - Hex - Telnet - bootloader (YouTube, broken file download links)
• Is T-Mobile modified Asus RT68 upgradeable to DD-wrt? - pg 2 (cdmarshall's post)
• Is T-Mobile modified Asus RT68 upgradeable to DD-wrt? - pg 4 (UKEE's post)
• Telnet Access to TMO Cellspot
• Flashing TM-AC1900 to RT-AC68U/P - Bay Area Tech Pros
• Flash ASUS TM-AC1900 to RT-AC68U - Angel's Place
• Does T-Mobile/ASUS TM-AC1900 (RT-AC68U) Support Merlin? (read the whole thread, if you like!)
• Asuswrt-Merlin on converted TM-AC1900 : Problems, upgrade path to 384.6
• Update converted Asus RT-AC68U (TM RT-AC1900) to the newest Asuswrt-Merlin firmware. Is it possible? - r/HomeNetworking
• The Badger v2.6.0 by Jason Buechler
• rebadging the TM-AC1900 to Asus rt-ac68u (and pushing unlocked firmware) using The Badger (may 2019) (YouTube)
• Flash T-Mobile Cellspot TM-AC1900 to RT-AC68U and install Asuswrt-Merlin - Quincy Huang (some links are broken)
• TM-1900 still showing after RT-AC68U firmware took (references a possibly deleted comment on Bay Area Tech Pros)