we have tested CONTROLLINO Maxi with SIM800L module and it is working. So I see no problem with SIM900 (aslo communication via UART).
Only issue with SIM800L was, that at CONTROLLINO is implemented current limitation at pinheader 5V output to 200 mA. During the transmitting, SIM module needs more. We had to power it from external PSU.
But because of SIM900 has own power supply, this is not an issue for you, so it shall work !