Weird Outlook Autodiscover issues

Going bald on this one.

- Windows Server 2012 R2
- Microsoft Exchange Server 2013 SP1 CU9
- Outlook 2010/2013/2016
- Behind a Cisco Firewall

Our set-up is 2 Domain controllers and 2 Exchange 2013 servers.

While I'm on a PC outside of the network that is not joined to my company domain, I try and create a new outlook profile and set up my mail account in Outlook. I get the error "The name cannot be matched to a name in the address list" when trying to autodiscover the settings. This is affecting all users.

I've looked at most of the obvious stuff:  

- Exchange Remote Connectivity Analyzer and Microsoft Connectivity Analyzer Tool finds no issues  
- "Hide name from address list" option is unchecked in ECP  
- DC's are in sync
- The Global Address List appears to be fully populated  
- In the "Attribute Editor" section of the user account in Active Directory, the "showInAddressBook" attribute looks correct and lists the default global address list  
- I deleted the entire %LocalAppData%\Microsoft\Outlook folder and deleted all profiles, and cached credentials on the client. I also tried it on several different PCs, none of them worked. Account that were already set-up continue to work.

This USED to work, about a couple of months ago when I set up the servers I was able to connect fine. Something changed between then and now. Any idea this might be?