How to configure an external static IP without using bridging
How to configure a static IP for Macvlan? I tried it and it can obtain an IP automatically.
When attaching to an external network you either will need to configure a static lease on your external DHCP server (your router if dealing with a home network), or you can configure a static IP from inside the instance exactly as you would do it with a physical Linux system.
Depending on what you want to achieve, you may also avoid giving a separate public IP to that Incus instance. The service of that instance can be accessed through port forwarding using proxy devices. The end-users will be connecting to the public IP of the Incus server at a specified port that forwards to the port of the instance.