Those do exist, they’re called cell signal boosters. Once upon a time, I believe, some American cell providers would give you one for free if you had bad signal at home, which mattered a lot more before phones all had wifi calling.

I think in the US there are even public 5G frequencies that can be used. In most of Europe you would need to buy an expensive license to do that.

Private 5G networks usually need internal eSIM cards, you can't just let public devices roam into the private 5G net.

Benefits of 5G over WiFi: much better roaming between APs, higher distances, and better congestion management if there are hundreds of devices connected to a cell.

I need a portable mini 5G cell tower in a backpack form factor so that I can have 5G service indoors no matter where I am.