Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
Let me preface this by saying, don’t do this. Don’t even think about doing it.

However.

You need to write down your external IP address, or better yet map it to a domain name using something like dyndns. Then you port-forward all of the relevant ports in your router to the time capsule. This effectively puts your NAS directly on the internet, without a firewall. Expect all of your information to be hacked (especially if it’s an older device) and anything you have stored to be exposed to the type of people who will exploit it. An especially evil person could even add files to your backup, so that when you restore, you restore with a virus. Like, really, don’t do this.

But then it’s just a matter of pointing your Time Machine restore to the IP address or URL instead of the NAS on your local network, and it will restore.
Thanks - I appreciate the detail and hear you on the complexity and security risk it creates. I'm not yet thrilled about this route. I hope something better comes along.

For now, maybe what I can do is iCloud backup and then have key folders that are accessed through a remote NAS. In this way, I can recreate the basics of my machine with iCloud's backup, which is built into the Apple ecosystem.
 
If you prefer not to use cloud storage and choose to back up several machines manually onto hard drives, you're really making things harder for yourself without a good reason. Cloud storage is affordable and much more dependable than the hard drives you can buy. They offer a lot of redundancy at different levels, which means they're very durable.

I personally use Restic as my backup tool since it's completely free, super fast, and very reliable. I've been using it for a long time, have restored many backups with it, and it generally has a great reputation. I also use both Storj and iDrive e2 as my backup destinations, adding my own extra redundancy just in case, because these services are so inexpensive.

If you're not a fan of using command line tools, I suggest trying Arq. While it might not be the fastest option, it's very reliable and affordable. You can set it up and forget about it. This way, you won't have to keep buying new hard drives and doing manual backups. Arq works with many different destinations, including all S3 compatible object storage services like the ones I mentioned. Another popular choice is Backblaze B2.
I like the look of Backblaze B2. Restic seems interesting. What is the learning curve?
 
I like the look of Backblaze B2. Restic seems interesting. What is the learning curve?

It's incredibly user-friendly, arguably the most user-friendly CLI tool for backups I've encountered and used. If you prefer not to use the CLI at all, there's also a free UI called Backrest.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.