VPN on Orbi without DDNS

I think maybe this is because your Orbi assumes that, with your WAN IP address being assigned by DHCP by the ISP, your VPN can't work. The most straighforward way to address this would be to get a reserved/static WAN IP from your ISP, which probably costs extra, if they offer it. If they don't offer it, or you don't want to pay, there may not be a good way to make it work. But you can probably make it work anyway, just with lower reliability. If you can determine your WAN IP address, subnet mask, and default gateway, you could probably manually enter them into the Orbi as a static IP address, and the Orbi should allow you to use the VPN feature without enabling the DDNS feature.

The problem here is you run the risk of your WAN IP lease expiring and your ISP assigning your connection a new WAN IP address. Because you will have disabled DHCP, your manually-configured WAN IP won't match your ISP's IP configuration, and you won't be able to use the internet at all from home, and your VPN won't work while you're away, either. You'll have to stay on top of things and check regularly, and update the WAN IP settings as necessary. It'll be free, and will probably work most of the time, but it really depends on your ISP, how often it changes WAN IP leases, and your tolerance for outages due to mismatched configurations. And, I suppose, your ability to remember that, during an outage, one of your early troubleshooting steps will have to be to re-enable WAN DHCP, get a new lease, then transcribe that new info back into the static IP settings field.