Question g900t

Dear Members

I have a quuestion, I have been trying to write a cert in a phone for 2 days now with no luck, as background I received a phone with a custom recovery, the customer wanted me to load the original stock rom and write a cert, I was able to flash stock rom but when I did a factory reset I got an error saying no efs, the phone booted normally then I repaired efs, now I dont get that error when I reset the phone but now randomly the phone reboots, I have tried different boxes and all give me an error, I cant connect it via UART even covering the battery connector since it is nto recognized by any box, my question is, is it possible to write a cert via download mode since I think that is the problem with the phone. I know in some posts they talk about at commands to write a cert, is there a good person that can share this knowledge so I can get this to work

Thank you so much