Even at its greatest extent, much of Eastern Europe was beyond Rome’s grasp. But of course, there were people there. So what was going on in the East at the time?
A similar question was asked some months ago by u/Pixelfireblade, without any answer.
I think an answer would begin with : "the Roman Empire stretched over 5 (or 15) centuries, it is indeed obvious that no sinlge answer could be given." (a real historian would frame it better of course)
Maybe by narrowing the period you could increase your odds of getting an answer.
I found two (far) related answers :
Why didn't the Romans conquer Poland or Eastern Europe? answered by u/bitparity and u/ComradeSomo
How far did the Romans venture into Eastern Europe? answered by u/IronDBZ