The use of "on" signifies that a person is connected to the network. For example, you may say that you are "on the internet" to indicate that you're connected to the internet. If you use the "on" preposition, I suggest: "Data charges may apply when on the mobile network." The use of the "to" preposition indicates that the object will be charged. The sentence "Data charges may apply to the mobile network" means that the "mobile network" will be charged. If you use "to", I suggest: "Data charges may apply to users connected to the mobile network."

"to" or "for" sound best. "networks"