It is really irrelevant that the Romans did not permanently conquer the area east of the Rhine. Of course there would still be commercial and diplomatic contacts. Plenty of money to be made for both sides.
The Romans didn't permananetly conquer anything, as their Empire finally ceased to exist -- and it only endured eighteen and a half centuries, from the Roman conquest of Ostia to the fall of Constantinople. They had camps and colonies in areas which often are not considered part of the Roman Empire. The commercial reach was enormous. Even in what is now Finland the iron industry owed its start to trade with Rome.