You know sometimes Apple has good ideas and sometimes they have ideas that are really bad. This is one of those. Someone at Apple thought it would be a good idea to add a setting to your phone, so that if your WiFi reception is not very good, the phone should flip to using cellular data.
That might be a reasonable answer, if everyone had unlimited cellular data but just about no one does. The problem is there's a setting in IOS 9 called WiFi assist, that's on by default.
You must go turn the setting off. There's no reason I can think of that anyone would want to have this setting on. To find it go to the settings on your iPhone, look for Cellular Settings and scroll to the very bottom. There you'll find the WiFi assist setting and ensure that it's off. Once you do that, you won't flip over to using cellular data when your WiFi connection isn't as strong as it might be.
Yes you won't get the best connection. But you also won't be eating up your data plan’s "Data" so that you end up with large bills. People have ended up with huge Cellular bills because of this. Don't let it get you.