Good question. We don't have any denial or confirmation that Sejun's work won't work on the 99th floor. So, Theo could bring Sejun's phone to the hunter leader he's associated with and then Sejun can talk with his family after a charge outside.
But then again, the first thing Sejun would've done the moment he stepped into the portal to the 99th floor is contact his family, and so the fact the didn't leads me to believe maybe the 99th floor is out of connection reach.
I don't know.
They could be using router for the wifi if it is a chain router Theo or iona could help placing routers on each floor and it is said that hunter phone is charged by sun so the router also could be. So i don't see reason why he can not connect to his family like that
.... just a thought... can't Theo bring mc the hunter phone? It's seems they can use wifi in the tower with it since could post theo's pics. Even if there's no wifi or mobile data working he can use video recordings or just letters between, so mc at least communicate with his family