If the Orbi has an AP mode, probably needs that. My guess the lan is static on a different subnet than the MX. The MX wants to do DHCP, but the Orbi isnt doing that on the LAN. You could try via the WAN, but then it probably won't be available. I would plug directly into the orbi lan and configure it to have a static private ip in the subnet with the MX. Then connect then 2 back up via LAN ports.