It's an old PHAT ps3 so I'm 98% sure it shipped with a FW lower than 3.55. So, what can I do with it?
If it old phat one, like you said should be lower than 3.55. Needs to be downgraded to 3.55,here is a table to tell you what you need to downgrade phat ps3:
Phat type ps3’s
CECHAxx (OK To Downgrade) NAND = Progskeet / TEENSY++ 2.0 / InFeCtuS
CECHBxx(OK To Downgrade) NAND = Progskeet / TEENSY++ 2.0 / InFeCtuS
CECHCxx(OK To Downgrade) NAND = Progskeet / TEENSY++ 2.0 / InFeCtuS
CECHExx (OK To Downgrade) NAND = Progskeet / TEENSY++ 2.0 / InFeCtuS
CECHGxx (OK To Downgrade) NAND = Progskeet / TEENSY++ 2.0 / InFeCtuS
CECHHxx(OK To Downgrade) NOR = Teensy++ 2.0 / Progskeet / E3 Flasher
CECHJxx (OK To Downgrade) NOR = Teensy++ 2.0 / Progskeet / E3 Flasher
CECHKxx (OK To Downgrade) NOR = Teensy++ 2.0 / Progskeet / E3 Flasher
CECHLxx (OK To Downgrade) NOR = Teensy++ 2.0 / Progskeet / E3 Flasher
CECHMxx(OK To Downgrade) NOR = Teensy++ 2.0 / Progskeet / E3 Flasher
CECHPxx (OK To Downgrade) NOR = Teensy++ 2.0 / Progskeet / E3 Flasher
CECHQxx (OK To Downgrade) NOR = Teensy++ 2.0 / Progskeet / E3 Flasher
DECHAxx (OK to Downgrade)