You should not have PC's attached to a WAN interface (at least not in any typical environment).
You should have two VLANs configured. One for the PCs and one for whatever the Cambium is. The MX LAN port should be configured as a trunk port to connect to whatever switch you are using. The switch will also need the same VLANs configured.
And where the Cambium's plug into the switch - those switch ports will also need a configuration to match whatever the Cambium needs.