ios 9 and iPhone 6 or 6s have a new feature called wifi assist. Turn it off.
THis feature uses cellular if wifi is insufficient. Apple failed to notify consumers how it worked and left it on by default.
Apple did actually advertise the feature in their software licensing agreement, except who reads those?
I argued many times to make sure Wi-FI Assist on iOS 9 was disable by default before launch. I see pretty design and ease of use takes precedent of sensible software decisions.
Oh I know. That is the pitfall of modern software design, and comes with the "patch it later" mentality.
Samsung at least on the subject of Wi-FI Assist, was thoughtful enough to not enable this feature on either my Galaxy S3, or Note 4 by default, although they did break the Wi-FI driver on both of these phones. They know exactly what they did and why with the driver, but haven't opted to fix it yet.
Let's see if Apple changes their mind come iOS 10. Apple had Wi-FI sense in some form enabled in multiple developer previews prior to iOS 9, but kept removing it from the final builds. I wonder why . It's still broken.