"firmly instructed" is a correct and usable phrase in written English.
You can use it to emphasize that someone gave instructions in a clear and determined manner. For example, "The teacher firmly instructed the students to put away their phones and pay attention to the lesson."
Birt politely but firmly instructed me: "Be systematic.
The owner gave him $20 and firmly instructed him never to return.
Although I thought this might be interesting, my family did not see martial law as part of the holiday experience, and I was firmly instructed to cancel it.
Then a woman from the funeral home firmly instructed everybody to "please return to your vehicles now," and we all complied quickly.
The trolls were firmly instructed that there should never be anything bad written about the self-proclaimed Donetsk People's Republic DNRR) or the Luhansk People's Republic LNRR), and never anything good about the Ukrainian government.
Had his captain, Mike Brearley, not firmly instructed the reluctant all-rounder to take off his trainers and put his bowling boots on for the session, Edgbaston may never have been inspired to reverberate so uproariously.
Of his performance in court -- enduring Mr. Hussein's finger-wagging assaults before calmly and firmly instructing him to behave like what he has become, a criminal defendant, and not like what he insistently told the court he was, Iraq's lawful ruler -- Mr. Juhi added, "I think I gave an example of what the law can be".
First, staff firmly instruct, mobile phones, cameras, cigarette lighters and all other light sources are to be stowed in a bank of lockers in one corner.
If anyone, be it an auntie or a schoolmate, asked, I was instructed, firmly and unequivocally, to lie.
Even small student misdemeanours are to be picked up on, and students are instructed very firmly to stand up silently at the beginning of the class, say "bless you" when a peer sneezes and "I'm sorry" when a teacher reprimands them.
As originally written, the rules instructed House members to firmly decline the honor.
