The Russian Federation currently has 21 regions called Republics. In 1954, when the Soviet Union transferred the Autonomous Republic of Crimea to the Ukraine Soviet Socialist Republic did they have the consent of the Russian Federation? What does autonomy mean? Can you imagine that if the United States was composed of 49 states and a Republic of Texas, that perhaps Texas was something special?
This is not like the case of the German-speaking Sudentenland of Czechslovakia which was never a geographically-defined region. The self-proclaimed Province of Sudentenland in 1918 was only a portion of what the German Empire later seized.
As far as I'm concerned, the status if Crimea is an internal Russian matter. However, if the Russians want to return Köningsburg and East Prussia back to Germany, I think we have a deal.
They offered; the Germans declined. It seems that Soviets used the Kaliningrad SSR as their toxic waste dump, and now nobody wants to clean up the mess.