Apart from the subnet conflict issue, I still can't have an Internet connection from the failover 4G router when the cable from the failing ISP is plugged on WAN. Yet, if I unplug the cable, I can get an Internet connection.
Any idea on how to make the router detect the failing Internet connection on the interface without physically unplugging the wire?
When the ISP's cable fails, it takes a certain amount of time to detect, and only after the failure is confirmed will the WAN network source be switched.
You can try to set the sensitivity to reduce the detection time.
The weird thing is, I lose any connection as soon as I plug the ISP's cable.
I tried Low and High (Medium being the default) but to no avail. No failover to the 4G router's connection 
Have you moved the priority of the connection to suit your setup? Towards the bottom of the multi-wan settings page you can move them up or down based on desired behavior
Here is my current setup
My desired behavior is Ethernet 1 (the currently failing ISP is plugged in here) has priority over Ethernet 2 (where the 4G router is connected) but only if Ethernet 1 is live. The problem is, the failover didn't work as expected: there is no Internet through Ethernet 1 and yet the router didn't get it from Ethernet 2.
If I unplug the ISP cable from Ethernet 1, I have a working connection from Ethernet 2 
What’s the WAN IP you get? Sounds like you may have some IP conflicts going on. Most cell providers use CGNAT and assign you a private IP. Check your logs look for errors during the time is should switch over
Which WAN IP?
If I unplug 1, I have Internet connection through Ethernet 2.
If I plug it back, I have no connection at all while I expected to still have a working Internet connection, still through Ethernet 2 as a failover.
Go read up on networking. If your subnet is /16 that’s the same network. Again it sounds like an IP conflict
I don't see why the two interfaces would be on the same subnet. The netmask from both (ISP modem and 4G router) dhcp servers is 255.255.255.0. And where would be an IP conflict if the two interfaces are assigned different addresses and different gateways to the Internet at large?
Really, this use case seems to be the closest to a by the book use case for the multiwan failover mode and still the manual is finally not so clear on how to make it work.
What do your logs say when it should be in failover?
We hope you can try to reproduce the problem again after the router is started and export the log to us.
Hi there,
The txt log file you sent is incomplete and does not contain much information.
It is recommended that you use SSH to execute the following command to increase the log level, then reproduce the failover problem again, and then export the entire log file package.
echo 3 > /proc/gl-kmwan/debug
Please export the log here, do not just copy a section.
I just sent two log files.
Could you please reopen the other thread as it was on a different issue (set priority not respected)?
We have received your log file. We will analyze it.
In addition, your other post has been reopened.
