I have the same exact problem as this post from Dec 2017, but the solution was not public. Any ideas what I can do?
We are having a issue with our billing partners posting back to our servers. I have gone into the firewall settings and whitelisted the billing company’s ip ranges but still no communication. I have also not seen any firewall events that would indicate the postbacks getting blocked. No errors are showing up in our servers or our billing partners. Page rules set for postback script dir disabling cache, security, performance…pretty much everything but it still does not now. Even in development mode postbacks do not work.