API of cnMaestro Cloud for SMS Gateway Integration

We are going to install a Wi-Fi network of 62 nos. of cnPilot E400 AP with Cloud controller option to reduce cost and service which in general requires in on -premise controller. Now, our client asking for OTP based authrntication for their guest users. Hence, we need the API as we need to integrate with our existing SMS gateway which is not listed in the options mentioned in the cnMaestro Cloud. Now, how can we tackle this situation with existing Cloud controller and SMS gateway?