You will only see the return for the month in which registration was granted. As your registration was granted in the month of August, you will only see the return for the month of August on your dashboard. As per section 40 of CGST Act, you have to report the details of outward supplies, from the date of liability to register till the end of the month in which registration was granted, in your first return (GSTR 1 of August) and
the details of inward supplies from the effective date of registration till the end of the month in which registration was granted in your first return (GSTR 2 of August).
Since you have registered timely, your effective date of registration is your date of liability mentioned in your registration certificate (GST REG-6).