That's like arguing that the southern United States, which seceded from the Union had a "right to self determination." Those southerners owed loyalty to the United States and hence the Union was right in retaking the south back by force. Just as the Russians living in Crimea and Donbass owe loyalty to Ukraine and thus Ukraine is right in taking back by force it's territory. Those southerners had a voice in the U.S. government before seceding and could vote, just as the Russians in Ukraine.
The Anglo settlers in Texas owed loyalty to Mexico; after all, they were there at Mexico's invitation. What right did they have to steal Mexico's territory - which had belonged to it for centuries - by seceding?