Using Putty (115200 baud), you must interrupt the boot sequence by pressing Ctrl+C when you see "Hit any key to stop autoboot." From here, you load the s20-firmware-fixed.bin via XMODEM. This is a last resort for hardware hackers.
If you followed the Recovery Mode steps above, the answer is . The boot loop, the constant reboots, the disappearing 5GHz band—all of these are software pathologies. The ZLT S20 is actually robust hardware (Cat6 LTE, 2x MIMO, external antenna ports). zlt s20 firmware fixed