@DemonSplicer I've done all the tests now, and this is what I get.
First setup LTE Router to connect to Mobile network with the APN you want, when you can ping the internet, step 2 would be to configure the EWAN.
Plug in the cable to your Fibre Router/ONT to connector "LAN4/WAN" on LTE Router.
Browse on router to "Settings", "Ethernet" and "Ethernet Settings".
Select PPPoE from the list and enter the details from your Fibre ISP. Then Apply. You wil see if you click on "Home" the network icon would be visible (and not the Signal Strenth anymore).
Do a SpeedTest, then you'll see your fibre Service provider name.
To test if the failover work, do a continious ping to say 8.8.8.8, then unplug the cable to your Fibre Router, and you would see the ping drop, and in a couple of seconds it would start pinging again.
If you have your Router's "Home" open, you will see the network icon is swopped with Signal Strenth.
Do a SpeedTest, and you would see the Mobile network name, then plug in the Cable to LTE router again, and in a couple of seconds, it would ping via Fibre again and Speedtest would show you that it work.
There is no "tick" to enable failover, and then the thing that irretate me the most, it only detect the connection with the ON state of the Ethenet Interface, so if a PPPoE username fail, or the Fibre drop, the Eth3 interface would still show as on, and the router will route traffic to the "broken" route.
It doesn't do a route test to see if the internet connection disappear, and then switch to LTE.
For it to work you need to unplug/switch off Fibre Router.
Hope it clears things up for you.
If you want this automatic switching, invest in a dual network card PC with pfSense or a propper Router that has the better features.