Something that I notice a lot is my iOS device remains connected to the home network when in the backyard parking lot, about to leave the house. The router is on the other side of the house.
The LTE reception in the back yard is very good. The Wifi connection is weak and causes navigation apps to exhibit poor network performance.
Desired behavior: Poor network conditions are detected and wifi connection is dropped in order to enhance application performance.
Unsatisfactory workaround: Install network access point near the back yard. This is not a satisfactory solution because all this does is change the geometry of which areas near the house exhibit poor application behavior on account of wifi remaining in range but yielding poor throughput. The goal would be to eliminate the poor behavior happening within these regions, rather than changing the geometry of these regions.
Another parallel to this is what happens when a network is not configured properly and your phone connects on Wifi but gets a 169.254.x.x link-local address when DHCP fails. I don't currently have a good overview of how Apple is handling this on iOS, but it obviously needs to fallback to cellular promptly.
Best Answer
There is a setting in iOS in Settings > General > Cellular -> Wi-Fi Assist, which uses the cellular data when wifi is bad.
However, this is on by default, so it may not be helping in your particular case.