Tried the app on two different phones with two different USA numbers. Never received the OTP on either. What if this happens when executing a transaction that requires OTP?
You'll just have to enter the transaction PIN (which you decide while creating the account).
Sim binding generally fails because the individual is unable to send an sms due to recharge/network issues. Can you raise a ticket with the support team? Someone will quickly get in touch and help you onboard.