How to Auto switch between 4G and WAN on connection?

I have a AR300M and E3372 4G, I would like the 4G connection to switch on and off depending on the WAN port being connected or not. How do I do this?

I have looked in ‘Load balancing’ and under 'configuration get the following errors

WARNING: some interfaces have no default route in the main routing table!

WARNING: some interfaces are configured incorrectly or not at all in /etc/config/network!

WARNING: some interfaces have no metric configured in /etc/config/network!

I am running the 3.04 firmware. (should I downgrade?)

Also I have set the wifi to 300 but never seem to get more than 144Mbit/s

Please use v2.264 firmware which has this function by default.

3.04 is testing firmware for AR300MD (dual band)

For speed, are you using phone? Is the phone 150M or 300M?

I have downgraded to the V2.264 as suggested, but I still see packets moving in the TX and RX of ‘tethering’. Also in the Luci ‘overview’ MWAN status both tethering and WAN are ‘online’

I have a cradlepoint ibr600 and when connecting the cable to WAN port the 3G goes offline and packets drop to zero, on disconnecting the WAN cable the interface comes back up.

I don’t think the ‘tethering’ interface goes down as I see a steady increase of TX and RX data which will add up over time particularly over an extended period where I want the 4g as backup.

re speed I was connecting with a Nexus 6p