Now I believe Hopper jumped into the Upside Down portal before it closed in order to avoid the deadly blast. Hopper has been in the Upside Down in previous seasons so it's quite plausible that he would hope Eleven could sense him and rescue him later; much like how Will was rescued in Season one. But Hopper somehow made it to the other open portal: In Russia. Now Hopper is being held in a prison where he was referred to as "the American" in the post credit scene. I believe it is Hopper because they have referred to him as "The American" the entire season. Plus, they would want to keep him alive because he is a valuable asset. He figured out how to travel from the US to Russia via the Upside Down. The Russia definitely want to know how so that their plans to invade the US can happen.