GL-A1300 Slate Plus / Howto convert 1 LAN into 2nd WAN port

Howto convert 1 of the 2 LAN ports into a 2nd WAN port, including the failover and loadbalancing features between the 2 WAN ports?

Try this method:

I do not work for and I do not have formal association with GL.iNet

Thank you for your quick response. Thís was not working as there seems to be no ETH2 device. Both LAN ports seem to belong to the same device ETH0, which is bridged with the two WLANs to Interface BR-LAN. ETH1 is belonging to Interface WAN.
This is the way I got the thing to work accoding to my favours: I attached an Ethernet to USB3.0 adapter (TRENDnet TU3-ETG) to the USB port of the GL-A1300. I installed the driver for the ASIX ax88179 chipset using the LUCI gui. Log shows kernel is recognizing the device as ETH2.
Standard admin GUI of the GL-A1300 is offering to connect the Tethering to ETH2, which I did, and have the full functionality there re. failover/loadbalancing and can shift the priority to the tethering device, if I want to.
This is my planned usage scenario, how I will use the devices in my mobile home: The GL-A1300 will span internal WLAN in my mobile home. It will also be used to prioritize traffic coming from either my Fritzbox 6850 5G using its external antenna conecctor with a 4 fold MIMO external antenna on the roof, or coming from the GL-AR300M16-Ext, also using an extermal 2fold MIMO antenna on the roof, should the campsite I will be staying at that point in time would offer a 2,4 GHz WLAN signal.
Would the GL-AR300M16-Ext be offering also 5GHz reception, I’d be even happier, but also need to confess, that there are not many campsites in Europe offering a 5GHz WLAN. There are many offering 2,4Ghz, though.