Yes, I have tried also with PhoneWakeState, but it was same story. Disconnected from 3G automatically, and not reconnected while invoking.
Now I tried this library, and it works.. I can reconnect while needed .. That's ok solution, thank you Erel ..
Btw. just for info: command .TurnDataConnectionOn did not work until this is added to manifest: AddPermission (android.permission.CHANGE_NETWORK_STATE) .. and that was not visible on first post, or within library ..