Is it legal to release a person who is an IDP and cannot return to the city...

In your case, everything depends on which article of the Labor Code the employer plans to fire you for.

You will also have the opportunity to appeal his decision, referring to the fact that according to the order "On approval of the List of territories on which hostilities are (were) conducted or temporarily occupied by the Russian Federation", the territory on which the enterprise is located is dangerous for work, because there are combat operations