Converting the WAN to LAN will not by itself solve your issue. The device is configured as a router and the LAN will have it’s own DHCP server and be configured to serve addresses in the 192.168.8.X subnet (out of the box). I would expect that you will have a conflict of some type with the primary DHCP server\LAN, which is probably on another subnet.

If the devices to be set up do not need to talk to the rest of the network during setup you can plug the LAN into the switch and NOT connect the MIFI to the main network. You would need to change the default IP address of the MIFI’s LAN to that of the customers network which would require a reboot of the MIFI. You may not need to make the WAN a LAN.

If you need to access to the balance of the LAN during config, then you want to convert the MIFI to a "dumb AP’ where you disable the DHCP server and change the LAN protocol from static to DHCP. The later will cause the MIFI and your wireless devices to get an IP address from the primary router DHCP pool.

You could leave the protocol as static and manually assign an IP from the customers subnet, but you would need to know this info. I consider the later a “safer” solution, as there is always an IP on the device and you can SSH in. Setting the LAN protocol to DHCP will leave the device with no IP if booted unconnected to a LAN as there is no active DHCP server.

The former can be done in the GLi GUI. The later I expect will require the use of the OpenWrt GUI, and may not be easy to do on a phone. If you do go down this route however there is a theme called Material which works reasonably well on a good smart phone (not so good on an old iPhone). Not sure if it’s in packages or if you need to find the forum link.