The installation of TWRP for klte and /e/ for klte dev (Pie) on a Samsung Galaxy S5 LTE - “klte” SM-G900F was uncomplicated for me. The device also did not need to be rooted.
But with devices from AT&T and Verizon the bootloader is very often locked or it can only be unlocked with considerable effort. You have succeeded.
Since Ubuntu 19.10 and Heimdall v1.4.2 don’t give you the desired success, I recommend you to try installing TWRP for klte with Samsungs Odin3 on a Wind°ws PC. Even if it’s not comforting, but many users have problems with Linux & Heimdall. Read more …