I would first try swapping the console cable to see if your current cable is somehow partially broken. And secondly make sure that you are using 9600bps, 8 databits, 1 stopbit, no parity settings on terminal emulator.
One thing you could also do is to connect keyboard and screen directly to appliance. The reasoning for this is that NGFW appliances have two different consoles: local console uses keyboard and screen directly connected to appliance, and serial console uses the serial port, and if somehow now the appliance is set to boot up to local console, that would mean that you can't control it via serial console. You can change and check which console is used in GRUB menu shown after reboot, and change the setting also with sg-bootconfig command.