@ElTom 

Unfortunately this solution doesn't work for me. 

In my case cable modem has subnet 192.168.100.0/24, DSL modem 192.168.99.0/24 and router LAN 192.168.1.0/24.

So all interfaces are in different subnets.

  @ElTom 

oh, I didn't know that there is new firmware - i tried with Firmware Version: 1.1.1 Build 20210723 Rel.64608
I returned device back to Dell... 

Can some one confirm it works, I will buy it again.

  @Vit100 I had this problem at first, here's how I think I solved it.

Make sure you specify external DNS resolvers on all ports, then restart the device.

I used the default configuration, where my provider's internet router says it will provide DNS service. You will see the DNS server being on the modem. For my internal network (LAN) of course I use external DNS but I had not thought of the WAN link's DNS service, and of course that is what "online detection" uses.

Then, even though the internet is "down" (disconnected coax or fiber), the policy of using DNS will say it is up. This is almost certainly the common failure mode in a home installation, because link status fails only when the modem is offline, and all my modems and routers are on the same circuit.

Using DNS is the automatic policy - and that is agressively cached in a normal consumer network - especially at the local device.

Instead, you have to use external dns (eg, 8.8.8.8, 8.8.4.4, 1.1.1.1) and the same for IPv6 DNS if you are using IPv6. Only then will the DNS request go to the internet.

Even when I did this, I had to restart the router. This doesn't make sense, but seems true. Probably they are capturing the DNS address for the health check and not changing it.

This is with firmware 1.2 on a 1.6 hardware ER7206.

FEATURE REQUST: Please Please TP-LINK: put a separate "ONLINE STATUS" along with "CONNECTION STATUS" on the main status page. It is VERY confusing in most environments to not see both status. Please?

OTHER FEATURE REQUEST: Show which of the WAN links are being used in what capacity. This would be with an "ACTIVE" status (vs backup if configured this way instead of load balance). If one is active and the other is standby, show that. If both are actively load balanced, show that. etc.

  @Vit100 

-> BTW, now researching router on raspberry Pi with link backup and load balancing.. Looks promising..

Any updates?

I purchased a tp-link ER605 Dual WAN Router to replace my RPi Home Lab Gateway.

See: Rob Ferguson dot org Blog How To: Use your RPi as a router

 Having the same problem, soo far no solution

  @Vit100 me too having the same issue !

  @Vit100 

Even I am facing the same issue , Primary WAN connects and re-connects ,

The ISP test their ONU device and found that the connection was stable but when connected to ER605

Hardware Version : ER605 v1.0

Firmware Version : 1.2.1 Build 20220512 Rel.76748

This Build 20220512 Rel.76748 may have bugs and TP Link must address at the earliest 

We just bought a ER7206 to do link backup, and have setup WAN1 as main (no loadbalance) and WAN2 as backup. 

What happens is that WAN2 becomes the default WAN after a while, for no reason, even after I have set WAN1 as main and WAN2 as link backup. 

Then when WAN2 goes offline, it wont switch to WAN1 and we endup with no internet connection.
After WAN2 comes back online, we still don't have internet connection since tplink doesn't detect its back online. 

I tried to setup 8.8.8.8 for "online detection" on booth WANs, same difference. 

I tried to create a Routing Policy to route all ports of all ips to all ips through WAN1: this finally makes the traffic goes through WAN1 for while. When WAN2 goes offfline, the traffic switches to WAN2... and since it's offline, no internet. (it's just ridiculous!!)

It kinda looks like the tplink firmware interprets WAN1 as being WAN2, since it switchs to WAN2 when WAN2 goes offline!! 

Finally I tried to turn load balance on and disable link backup. The same thing happens again! As soon as WAN2 goes offline, all traffic goes through WAN2, and no internet connection (even WAN1 being still online!!)

So, basically, ER7206 does worse than without it, since the only reason we bought it was to make sure we had internet access in case one link goes down!
But now we loose internet connection when the 2o link goes down, no matter if the 1o link is online!!! 

why does this product even exist? 
 

  @rhradec 

Good question.. but for me it doesn't exists anymore, returned back as soon as discovered this problem..