Whether yo put your Orbi gear (never had to deal with that kind of device) in something like bridge mode, and MX gives DHCP to devices behind Orbi gear
Or
Create a transport subnet (ex 192.168.100.0/30) between MX interface and Orbi interface
Put 192.168.100.1 on MX interface facing Orbi
Put 192.168.100.2 on Orbi interface facing MX
Create a static route in MX:
to reach subnet 192.168.1.0 (Orbi), go to 192.168.100.2
Create a route in Orbi
to reach (whatever you need that is managed by MX), go to 192.168.100.1
you cannot have 192.168.1.1 on MX and Orbi