How to get my iPhone to disconnect from WiFi

When I'm outside in my yard, Safari just doesn't seem to be able to connect to the internet at all. And I think it's because Settings > WiFi is still showing it as being connected to my WiFi in the house, whereas it's really not. Or possibly the connection is so weak that Safari just can't use it. But it's apparently not weak enough for the phone to let go of the connection it thinks it has. So my cellular connection never takes over from WiFi. So -- no WiFi, no cellular connection - the phone is stuck in no man's land and I have no internet connection.

In this situation I've been manually disconnecting from WiFi and then I finally get a cellular connection. But then I have to remember to reconnect to WiFi once I get back in the house. It's not an ideal solution.

Until I can spring for money to buy a WiFi range extender or other hardware, what are my options?