Vacant IRS Mailing Address

I recently received a notice that asked me to mail back missing forms, 1310 specifically. The instructions said exactly "Please complete, sign and return the form to us at the address shown at the top page of this Letter". The address listed is Stop 6800 (TPR) Kansas City MO 64999. However, USPS returned this letter stating "Return to sender, Vacant, Unable to Forward". I've been calling 800-829-0922 for the past 3 business days, but it keeps saying no one can assist due to high volume.

Any advice on how to proceed?