Russia has defenses, but no ability to move forward.
You don't play RTS games, do you? The fun thing about a strong defensive line is that you can kill a whole lot of their guys for every one of your guys that they kill, and if you have enough guys they're going to run out long before you do.
What happens to those insecure areas?
The Nazis probably genocide the Russian speaking Ukrainians that live there, either by driving them out using terror, or just killing them all. Probably a combination of both.