The Wagner group are mercenaries in every meaning of the word.
Unpaid foreign volunteers in UAF are inducted into the UAF and subject to Geneva Convention and laws of war.
The (illegal, unrecognized) Donbass paramilitary will regret this.
I'm not so sure about that, based on what the Geneva Conventions say about mercenaries.
Given that said foreign volunteers were not "nationals of a Party to a conflict nor residents of territory controlled by a Party to the conflict", nor were they "sent by a State which is not a Party to the conflict on official duty as a member of its armed forces", it would be reasonable to deem them as mercenaries, and therefore have no right to be deemed lawful combatants, nor are they owed POW status.