SM-T810 stuck in "D/L- do not turn off target." Flash stock rom to fix?

On my rooted T810 I mistakenly hit restart through Knox and ended up in Odin mode. If I do Power + Volume Down it'll show the Samsung logo for a second, then immediately go to "Downloading...do not turn off target." Other key combos like recovery mode don't seem to work for me (unless I'm doing them wrong). What should be my plan from here? I'd like to unroot at this point, so try to flash a stock rom through Odin?

Thanks for the help.

TWRP: File is bigger then Target device

Some how it double posted im sorry.

TWRP: File is bigger then Target device

So i need to flash a cust.img through recovery because fastboot doenst work gives me "remote:command not allowed fastboot" but if i want to flash it through the recovery it says "Error: File larger then target device" i basicly only need to flash the cust.img to fix my phone.

What i tried:
Use fastboot, tried new version of android SDK
flashed diffrent recoverys, same results.

Please help i really need my phone ;/