No problem, the TNG RFID system will detect the TNG RFID Tag to charge the toll fare and not the mobile phone. Having no signal or no battery power on your mobile phone will not affect your TNG RFID Tag usage.
Below are the articles that may be useful for you :
At which toll plazas can I use the TNG RFID Tag?
What are the differences between SmartTAG and TNG RFID Tag?
If I have both SmartTAG and TNG RFID Tag in the car, will I be charged twice?