Issues with Applying for IDR plan

I applied for the IDR plan on the studentaid.gov website and I expected to just submit my tax forms and IDR application online because my sibling went through the same exact process by just submitting everything online. However, when I applied for the plan, it required me to physically send my tax forms and IDR application to a NELNET office. Annoyed, I still sent it over using the address that was given to me:

NELNET

PO BOX 740283
ATLANTA, GA 30374(0283)

I waited for over a week and found out that my mail got sent back? I'm not sure what exactly happened because they didn't even bother telling me why they sent it back. Is the address wrong? Is there something I am missing?? I have tried calling Nelnet but I keep getting put on hold for absurd amounts of time. please help! any help is appreciated!