Hello everyone! Thank you very much to Nick Justice and andregrillo.
The first time I cleaned the logic board using isopropyl alcohol in the region of the RAM chips, applied solder flux and tried to heat it up with the heat gun. It worked, turned on and stayed on for about 5 minutes, then it turned off again, returning the beeps.
In a last attempt I decided to try to put it in the oven as you suggested, I left it there for 13 minutes at 180-210 C.
Worked perfectly! I've been using it for 1 day, and I haven't had any more problems!