What do you mean with "control"?
If you are not planing on going online with the PLC software, an easier alternative to view and modify data is to remotely access the related HMI.
With the Red Lion's HMIs, for example, you can remotely go online with the HMI screens, so you can see and modify the values that are accessible to be shown/modified.
If you use a VPN router like StrideLinx, you can have an application for your phone to access the HMI.
I know there are other HMI and VPN brands around that can do the same.
In summary, PLC -> HMI -> VPN router -> smartphone