Wireless vs data connection Assume there is a 4G access point or router for internet connection, e. g. Dlink 953. That means a sim card is inserted to the access point. So the internet is routed via the data connection defined on the sim card. Users then connect to this access point via the wireless connection. In other words, they turn on the WiFi and connect to the access point. That seems to be fine. 

Now, the problem is that users are actually using a data connection (internet is available on the sim card in the access point) but their phone won't notice that and it seems that you are connected to Wifi.

What is the actual problem then? Some applications have setting for reducing the internet usage on data connections. In this scenario, we see that phones are connected via Wifi and they won't reduce the internet usage. For example, Instagram has a setting to reduce the amount of download on data connections and not Wifi. Since the phone is connected through Wifi, Instagram assume it is fine and won't reduce the amount of data. But, really the internet is provided by a data connection (sim card in the access point).

Any idea for that?
Recent Android versions have a setting to let you designate some Wi-Fi connections as "metered"; i.e. they're actually a hotspot for which you get charged per byte transferred.

On vanilla Android, go to the Settings app, then Data usage, then on the overflow menu, choose Network restrictions. The name isn't very obvious, but the next screen explains:


  Metered networks are treated like mobile when background data is restricted. Apps may warn before using these networks for large downloads.


Within apps, any "only on Wi-Fi" options are affected by these settings: the networks you select do not count as Wi-Fi for that purpose.