ZeroTier is an open source software to create VPN (Virtual Private Network) over the internet with 256-bit end-to-end encryption. It's reliable, no hassle to setup, and you can use it for free for non-commercial use cases. It's no surprise that ZeroTier is very popular among gamers and many homebrew projects alike.
Normally, you should be able to install ZeroTier on Linux very easily. However, it's a bit more tricky on openSUSE, as the install instruction on the official download page, won't work. See ZeroTierOne issue #1517.
Adding the right repository by opening YaST Software Repositories app, then click the Add button to add a repository.
Choose the Specify URL... option to add a repository via URL, then click Next.
You can enter whatever repository name that you want, but to prevent any confusion, I recommend you to use zerotier as a repository name (you're adding zerotier's repository after all). For the URL field, you'll have to add the official ZeroTier's repository for RHEL9:
Follow the onscreen instruction. YaST will prompt you about the unsigned file, which is pretty normal when you try to add a non-openSUSE repository, you can ignore it as there's nothing wrong with ZeroTier.
At the end of the process, you should have zerotier repository Enabled and Autorefresh.
Now, you should be able to install ZeroTier from YaST Software Management app. Searching for zerotier-one package to install.
Initialize ZeroTier's daemon by:
sudo zerotier-one -d.
Open YaST Services Manager to check whether your
zerotier-oneservice is currently in an active state, and also set to start on boot.
I hope this helps.