Unable to supply FONA 32u4 3.5 v through circuit
This is not something that should be tested on a breadboard.

The GSM module SIM800 requires peak currents up to 2A.

At least the battery must be connected as close as possible to the FONA power supply pins. 

I doubt that the breadboard pins are rated at 2A and you have six breadboard connections between the battery and the FONA supply, the voltage losses are to high.

Figure out something to get the battery closer to FONA power pins