Were the devils who served the Prophet Solomon, peace be upon him, disbelievers, knowing that the word 'devil' is applied to disbelieving jinn?

The devils (shayatin) are the rebellious and wicked among the jinn. There are also devils among humans. Both categories include disbelievers and transgressors. Allah Almighty subjugated all jinn—their believers and disbelievers alike—to His Prophet Sulaiman (peace be upon him). They obeyed his command and could not disobey him. Whoever among them deviated, Allah would make him taste the torment of the Blazing Fire. This was a subjugation from Allah, Glorified be He, and no one among humans has ever been obeyed by the jinn in absolute obedience except Sulaiman.