Sorry Ben,
A citizen of his own country when attacked by another invading force even when not in a military status and uniform cannot be an illegal combatant. He, by implicit and explicit fact of nationality, has a duty to defend his country. He may an irregular, but not an unlawful combatant.
Just look at our oath of citizenship, “ support abd defend the US constitution...”.
Born citizens implicitly have the same duty as an immigrant who explicitly takes an oath of citizenship.
Agree with that definition of legal/illegal combatant in the law of war.
In 2014, a Color Revolution in Ukraine illegally overthrew a Democratically Elected President. 3 Oblasts (States) immediately declared independence and/or affinity with Russia. Since that time the current Ukrainian Government has waged war against the breakaway Republics. Russia has formally entered the conflict