How to force nokia 520 8.1 to only use data for mms?

[Zodemere]

Ive got at&t prepaid unlimited mms but when data is turned on to send mms it uses data for unknown things. Ive got email sync off and no apps running in background. Is there a app or custom apn settings that would help?

 

[Zodemere]

Ive now noticed simply turning data on and then off will use 1 cent! Why is the 520 accessing data when im not using any apps or etc?