how to reach airport time capsule away from home

Extremely difficult unless you have a "simple" modem......the kind with only one Ethernet port.....and you have the Time Capsule connected directly to the simple modem and set up as the "main" DHCP and NAT router on your network.

Even if you have the right ingredients, it will take a lot of time to try to get things working....with no guarantee that things will be successful.

And, you won't be able to connect from some networks because of restrictions on this type of connection, so even with the correct settings......there will be times when you won't be able to connect back to the Time Capsule from a remote location.

There might be a reason why Apple never supported this type of connection.

Finally, not sure why you would want to do this, even if it might be possible.