@Trigger77 I discovered there's an IP conflict, as follows:

[osg:C0-06-C3-3D-15-26] failed to obtain the IP address for WAN/LAN1 because IP-Address/Gateway conflict with IP-Address of another LAN/WAN ports (IP-Address=192.168.0.21, Mask=255.255.255.0, Gateway=192.168.0.1).

The 192.168.0.1 is the gateway address of my router with the second WAN. This is the same address as the ER605. 

I tried to change the address of this router to 192.168.0.2, but it hasn't worked. The same error log appears, but now with the 192.168.0.2 gateway address.

What could I do?

@Virgo thank you, it worked!!

@Virgo @Trigger77 can you explain a bit more about what you did to get it working?  I'm having the same problem.  I used the WAN configuration below and it indicates that I successfully connected, but I am still unable to get out to the internet.  Any idea what I've overlooked?

@Chadrock I had to change the IP Address of my second router because it was conflicting with the ER605. You can check if that's case on the logs page (System Tools > System Log). Then you have to go to your router that is conflicting admin page and change its address to a different IP range. In my case, I changed my second router's address to 192.168.16.1 (the prior addres was 192.168.0.1, exactly the same as ER605).

@Trigger77 I see.  Got it working now, thank you!