If you have data plan with a cap, it’s probably a good idea to dive into iOS 9’s Settings and turn off WiFI Assist.
The feature switches an iPhone’s data connection to the mobile carrier when it’s experiencing shoddy WiFi. If you have an unlimited data plan, it’s great –it ends the need to turn off WiFi when the local connection goes south and you can no longer load that hilarious goat video you found. It’s not so great, however, when you have a pretty low data cap and it switches from WiFi to cell without your knowledge.
If you’re part of the latter group, head to Settings > Cellular then scroll all the way to the bottom of the page to turn off WiFi Assist to keep your mobile carrier from taking all your money on your next bill.