To use the mAadhaar app you can simply download the app and set a password, and then enter your Aadhaar number; alternatively, you can scan the bar code on the Aadhaar card instead of entering the number.
Once you enter the number, the app will automatically send a SMS to the linked phone number. The OTP is entered automatically in the interface, and you cannot type it in.
However if the app does not read the OTP automatically, UIDAI suggests the following:
Go to the settings
Go to permissions control
Go to Apps
Go to mAadhaar app
Allow the read SMS option