Agree with the previous answer, but may have something to add.
After recently rewatching the series, I started to wonder about the nature of The Upside Down.
When Eleven first astral projects into the presumed dimension, the dimension itself appears completely back, except for Eleven herself and that we can see she is walking on a floor of water, which may be a "reflection" of her corporeal self in the deprivation tank manifesting into her astral projection.
Now the story plays out that Eleven eventually gets so frightened when she comes across the monster again, that she literally blows a hole between the dimension and the Lab. That entry space becomes a "mix" of moldy spore residue and growths, damaged "fabric" of space-time, and pre-existing parts the lab.
So my guess is that when this hole was torn open by Eleven and it "touched" the characters' reality via the entry way in the lab, it then quantum entangled itself and created a "mirror universe" of their reality. This is how Will can try and "hide", because he knows the reality better than the monster...
So Will can contact his reality, because those communication devices now also exist in The Upside Down--he called on it's version of the phone in his house.
However, some of this stuff may have to furthered by Will getting exposed to the spores and radiation that perhaps made him like Eleven (and granted we don't yet know how exactly Eleven became kinetic--(the kids actually debate if she's like an X-Men character or not.) and then also in science-fiction (Fringe, Star Trek: Discovery are recent examples) "spores" themselves seem to come with kinetic properties and/or networks for quantum entanglement. Also Will can seemingly astral project into the mirror reality now too...
It's also true that there is a portal in the woods that we don't understand it's existence, but suggests that this may not be the first time the realities touched. We also know that Eleven is named because she is one of several "experimentees" that was either used or cultivated for weapon advancements. AND that she was not the only person who ever existed in the Monster's dimension, as a man appeared, talked to her, and evaporated. It's unclear if he was "currently" there or a preserved "memory" that Eleven could tap into.