Mofi 4500 Cellular modem but GL-750 slate when USB Tethering

As much as possible I use my Mofi 4500 with a T-Mobile SIM for my RV’s internet connection. I use my 750 Slate for wireless device connection using Access Point using the 5G Wifi.

My iPhone service is through Verizon. When my T-Mobile service isn’t available I would like to switch to the Verizon USB tethering on the 750 Slate.

Is there a setup that would allow devices to always connect to the Slate but use the Slate’s USB internet when the phone is connected and the Mofi when phone is not connected?

So you want to use Slate to connect to your Mofi via wifi and connect via USB to your phone for Internet.

If you do this, it will use Mofi first and fail over to USB tethering. If you want to reverse the order, you may need to change the metric value in /etc/config/network and /etc/config/mwan3

A lower metric means priority.

I currently have Ethernet connection to MoFi, so I need to change to Wifi connection and it will automatically work?

Yes just try setting repeater.