It seems like a good idea that if you don’t have a good wifi connection your iPhone or iPad will switch to using your mobile connection whether it be 3G or 4G.
The down side to this is if it is doing it automatically you might be using your mobile data allowance when you least expect it because you thought your were on wifi. Not great if you are doing some software updates or watching videos that can use up a lot of data.
This is a feature that was introduced in iOS 9 called Wi-Fi Assist and it is on by default, I noticed that although I had this switched off when I upgraded to iOS 10 it was on again. Thankfully you can switch it off. [The wifi icon does switch to the cellular icon 3G or 4G, however, Apple should give you the option to display a popup message if it is switching to or from Wi-Fi Assist].
Before you do you should know it will not switch to Wi-fi Assist if you are on data roaming, it does not download data for something running in the background and it does not stream data for audio and video when using third-party apps or download attachments in email, again from a third-party app. It doesn’t mention if it does for one of its own apps so I would assume it might.
It also is not a feature on the iPhone 4S, iPad mini (1st generation), iPad 2 or iPad (3rd generation) with cellular option. So if you go to turn it off and it is not there you know why.
All you have to do is go to Settings on your iPhone or iPad, tap on Cellular and scroll right the way to the bottom, this could be a long list depending on the number of apps listed that could use cellular data, when you get to the boot on off the list just switch off Wi-Fi Assist.