I tried to follow methods 2 and 3 outlined in this guide https://discussions.apple.com/docs/DOC-3413 on accessing my Time Capsule hard disk from outside of my network.
I suspect it may have something to do with the fact that I also use my Time Capsule as my primary router.
More details below:
For the Method #2 in Setup section I skipped steps 1-2 because I have a static IP address and thus I don't need a dynamic DNS service. I continued with step 3 "Enable file sharing" & "Share disks over WAN". Then I followed the "To access the AirPort Disk:" section and tried to connect from another Mac connected to the internet using my iPhone data connection. When I tried to connect from Finder's "Connect to Server" and input
afp://mypublicstaticIPaddress:8888I got the "There was a problem connecting to the server" message.
For Method #3 I followed the Setup section to the letter and when I tried to connect as per the "To access the AirPort Disk:" section, which I also followed to the letter I got the same "There was a problem connecting to the server" message.
Can anyone suggest what else I may need to do to get either of these methods working?
UPDATE August 30, 2019: now, when trying to connect from Finder by pressing CMD+K and not inputting any connection type prefix like afp or smb, I get a different error: "This file server will not allow any additional users to log on. Try to connect again later."
UPDATE January 11, 2020: The issue was that I didn't have a public IP address, my ISP uses carrier grade NAT, so I had to ask for a public IP address.