I've been wanting to install openwrt in my Asus DSL N14U router that I have from an internet company in spain called "Pepephone" than later was bought by another one called "MasMovil".
I've finally managed to access the router in "rescue mode" following this guide, but pluging the rj45 into the last port (number 4) and I flashed the openwrt image with Asus Restoration Tool.
The flash was not successful theoricaly it get to the 100% but the router continued blinking like it was on rescue mode. I restarted the device anfter 30 min of blinking and the firmware that loaded was the same as before 1.1.1.6 from "Pepephone".
I tried to use my raspberry pi with debian to use tftp to flash it and it was aparently successfull too it sent a lot of data and it recieved it, but after 30 min it continues blinking and does not restart into the new firmware.
After that I tried to install the official Asus update image that has .trx extension and it worked perfectly well.
The problem is that I wanted to try OpenWrt, I don't use this device anymore, but I wanted to check the capabilities of this Firmware.
If it is throw away as you say you may try to "debrand" to ASUS firmware first. Using latest recovery tool. For a surprise it even got update last year https://www.asus.com/es/supportonly/dsl-n14u/helpdesk_bios/ (guessed your location from provider)