I'll admit, it's pushing it to have him survive there, but it could be done. It's a wholly unexplained realm. I mean, some of the buildings have working LIGHTS. We honestly know next to nothing about it. There could even be people there, in hiding
Mostly I'm just saying that if Hopper is survived at all, where else could it believably be? It's entirely possible he's gone, but I see it being hard for the show to move forward without their male lead, & something of a glue for the show, especially this show, which owes so much to its cast dynamic, & I think it's unrealistic that he'd have been moved to that Russian holding cell, so easily after the events that happened.
I doubt it's him in that cell, & if it turns out to be him, that's a bigger jump the shark moment IMHO, because this part of the story takes place in a fictional version of our world, & our world has rules, logic, history. It's absurd to suggest Hopper got whisked away to a Russian installation, so soon after everything went to shit for them in Hawkins
Whereas, the upsidedown can have its own logic, written specific to their story, as of now. They have a pretty clean avenue for creativity down there, if they use it wisely
For me, him surviving at all is pushing it (not to mention totally undermining his incredible exit). But if he's going to survive, spending a year bush camping in the Upside Down is the worst possible option. Him being the guy in the russian cell is the second worst, unless the Russian military has teleporters or some weird connection between their systems created a second portal in Russia for just a second and he was blown straight through the Upside Down to Russia (which would still be terrible).
The best possible option is probably that someone (Russian or unscrupulous Fed) dragged him out unnoticed and held him prisoner somewhere in the US.
The upside down is only an option for me if you reveal something totally game changing. Like, say, the upside down really is the future or has totally screwed up time in different areas, so that he finds a way back after a week or so but only arrives months later. Or maybe the Mind Flayer has a use for him and a way to keep him in some sort of suspended animation.