The first payment is due in October, so the bill should be ready (it’s just not “due” yet). Contact the servicer to find out what is going on. This is from the federal website: 6 Ways to Prepare for Student Loan Repayment to Begin Again – Federal Student Aid.
This has information, too, and it looks like billing might not match up with announced dates? When is your next federal student loan bill due? How to figure it out