As you do not have any medium to get OTP, you need to visit your Jurisdiction officer with proper documents such as a request letter for password/username / mobile number /email change, valid ID Proof, etc.
To know your Jurisdiction.-
1) Go to Gst.gov.in
2) Click on Search Taxpayer
3) Put your GSTIN and captcha code, enter.
4) Then you will see your Jurisdiction in red mark.
Officer will give you a temporary user id and password. After logging in the first time with the given user id and password you are required to change user id and password.
Hope this helps! For any further query feel free to message.