@Smok I cannot get mine to work no matter what.
I installed it before I had my 5G modem router preparing for the 5g router as a backup WAN. The second I connected the LAN cable from that router, the entire internet went down.
I then activated the sim etc - and now the internet returned - but preferred to route traffic to the slower 5g (like 250/25 where my cabled wan is 1000/100 (measured 850/125)).

I have tried:
- interchanging the WAN - WAN2 cables (wan2 = LAN/wan port 1). That made traffic go to the other port but thru the same router
- enabling load balancing using measured bandwidth - that makes traffic distribute like 50%-50% (expected like 75%-25%)
- having online detection to auto (initial choice)
- trying a recurring tip here to set online detection to manual on both wan-ports and ping 8.8.8.8 (dns 0.0.0.0)
- this tip disabling application optimization - or enabling it
- having enable load balancing checked and unchecked

The only way I have successfully managed to make all traffic go to the primary wan-port is to disable the secondary wan.

At best I can bring the traffic to the backup WAN down to like 50% (when enabled)
My primary wan is faster and flat rate, the secondary is on a plan (and only 1/3 of the speed).

I have seen the router table has both routers LAN-IP set to 0.0.0.0 could it be that the traffic is always routed thru the first hit in the routing table?

  @Smok OK, I will do that. Just thought You could tell me what setup workes for you.
Your tip apparently did not help make my attemps working.

  @Virgo   This is somewhat true for my scenario where I have only one active WAN with USB Modem acting as a backup - I am aware of that but that was not my point.

1. BUT even in this scenario clicking on "Application Optimized Routing" should not silently break WAN connectivity !  Either this choice should be inactive or there should be an (i) icon with explanation not to enable this feature in this scenario. when there is only one active WAN link and a second link is only a backup.

2. SECONDLY - what about a scenario with TWO primary/active WAN links and a third backup link ? In his case "Application Optimized Routing"  seems desired - but can it be enabled in such case or will the Link Backup feature be broken again ?

Do you agree with the above ?

My reason to post the original report was also that I found that there are many articles and discussions on the Internet about this "Link Backup" feature in Omada and most of these articles say that the "Link Backup" feature is broken, does not work, use policy based routing instead etc. Only some tutorials and videos mention that Link Backup actually does work (or can be made to work) but then again they do not mention that enabling "Application Optimized Routing" will break it completely. So there is some communication gap towards the user base for this feature. And I believe that the behavior which I identified is rather a bug not a configuration error - as explained in my two point above.

PS. Well, my setup works now so I do not care that much - but does TP-Link care about other customers who struggle to implement "Link Backup" and give up, saying it's broken ?