GSM SIM900A battery power supply circuit
Before you get too far into designing your circuit, you should calculate your power budget. Your modem draws 2 amps at 5 volts. That is 10 watts of power (2 amps * 5 volts). As JRE recommended, a switching regulator should be used. If you allow for 90% efficiency, you are now up to 11.1 watts (10 watts / .9).

Your battery can supply 1.2 amp hours at 12 volts. That is 14.4 watt hours (1.2 Ah * 12 volts). If you now divide that by your 11.1 watts of power consumption, you can estimate ~1.3 hours of operation. If the average power consumption of the modem is less, this operating time can be extended.

If this is sufficient for your application, start looking at 12 volt in, 5 volt out switching regulator designs. To avoid damaging your battery, the regulator should drop out when the battery reaches ~11 volts.