"should I change something in the sketch? "
That's what I am suggesting.
Treat the data coming from the serial monitor as ASCII characters, so instead of hex bytes coming over, such as
F0 00 04
instead you may be getting
'F' (or 0x whatever F is in the ASCII table)
'0' (or 0x30)
'0'
'0'
'0'
'4' (or 0x34)
which you can use as is for some things, but you need to convert to hex for others
But like I said, add a line to repeat the characters back
Serial.print(receivedByte, HEX);
and confirm that.