Comment by SahAssar

In "Frankfurt-Hahn" the "Frankfurt" is still the same Frankfurt you (probably) wanted, you just got a different airport located further out. "Frankfurt an der Oder" is on the other side of Germany on the border with Poland, a ~7h drive away.

So this is more like you going to NY and landing at Newark when you expected JFK where Frankfurt an der Oder would be like ending up in New York, Iowa.