inner join shows more entries
This part of your join is incorrect:

Trelaciones.Device_SN_O="FCQ1527Y0B3"


That should be your WHERE clause. 

Your join should be:

FROM Trelaciones 
INNER JOIN dispositivos 
ON  Trelaciones.whatever_key_column = dispositivos.whatever_key_column


So

FROM Trelaciones 
INNER JOIN dispositivos 
ON  Trelaciones.whatever_key_column = dispositivos.whatever_key_column
WHERE Trelaciones.Device_SN_O="FCQ1527Y0B3"