The IP which you have specified here, is of Author, right?As you said,
all of your outgoing traffic goes through PAT, seems like the publisher
is not able to communicate back to the author due to this network
restriction. Can you please test your FDM submit service on the
publisher and check if it working fine?
Have you configured AEM DS service settings on
Please refer to the sample PDF below which has a button to
add/remove/open the attachments. It would work in Acrobat Pro, but to
make it work in Acrobat Reader DC you need to Reader Extend the PDF.