I've found the link below which provides useful info on how to configure Time Capsule for remote connection. I've followed this and am able to connect to remotely manage the TC (via Airport utility) but still cannot connect in order to access files. I'm trying from a Windows PC.
The first thing to check is if the Time Capsule was told explicitly to share the files over the internet using bonjour or simply sharing to the WAN.
Have you verified that the active profile on the Airport is set to allow this?
My example has these turned off (you can see them under the guest access control)
Make sure that you have a clear path to the Internet in terms of firewalls, and ensure that TCP port 5354 and UDP ports 4500 and 5353 are open and unrestricted.
Once you've done that, make sure you are logged into iCloud on your computer, and that the remote Time Capsule is on (also making sure those ports are unblocked on that end). Your disk should now be available via FTP and AFP.
If you have access to a Mac, try these steps from the Apple Support Article about troubleshooting Back to My Mac.
Note: Perform these steps only if you've verified the information in the previously listed troubleshooting section is configured correctly.