There's a bench about 200 yards from my office that I'll sit and work at a few times a week. The trouble is that it is just close enough to hold a really poor WiFi signal. When the iPhone has a WiFi signal, it is pretty resistant to dropping it, even when you'd be better off on LTE. My solution is to give the WiFi a kick. I just flip up Control Center and press the WiFi button twice, essentially toggling it. Even though the WiFI is off only a fraction of a second, the office signal is so weak that my iPhone knows better than to try and connect and leaves me on LTE. If I just turn the WiFi off, I inevitably forget to turn it back on when I return to the office and end up using cell data when the phone stops connecting to WiFi. By toggling, I temporarily drop the WiFi but get it back when I return to the office.