Need help unlocking bootloader

Trying to unlock att's bootloader on my sm-n910a. I purchased it used and already have the dim unlock finished. I have scoured the interwebs on how to install twrp and even thought about downgrading from 6.0.1 to 4.4.4 to root then install twrp. Nothing seems to work since the bootloader is locked. I had this issue on my s4 back in the day and was able to root using terminal emulator but I can't remember nor find the process to force installation of super user. Anyone who can help please do, I'd greatly appriciate it.