@borgph 

It seems like here is the double NAT in your network, and if the ER605 connect to the main router's LAN port, then the main router's LAN IP couldn't be same as the ER605's LAN IP.

Because the ER605 WAN IP should be different from the LAN IP. Otherwise here will be the IP conflict.

  @borgph 

Would I be correct in assuming your architecture has the LAN ports of the Main, ER605 and OC200 all at the same level, and that you have different WAN connections for the Main and ER605 routers?  If so, then yes, it should be possible, but it's not by default (I had the same issue, but gave up and just changed to 192.168.0.x)  However, after becoming more familiar with the Omada ways, I think if you redefine the default gateway for the default LAN subnet on the controller (Settings->Wired Networks->LAN) to be 192.168.18.10/24 AND you change the fallback IP and subnet of the Controller itself (Settings->Controller Settings->Controller->General Settings) to be say 192.168.18.11 and netmask of 255.255.255.0 (or whatever is appropriate) then the OC200 should boot up on the right network and find the router and adopt it.  

Normally I'd test before proposing this, but I am live at the moment and cannot mess around.

  @borgph 

Good on you for finding a solution.  My issue is, if you ever factory default the router, you have to redo all this with a local PC...hardly an enterprise grade solution IMHO.  And that's most of the reason why I kept my management addressing as 192.168.0.x....if you can't beat it, join it.  @Fae Maybe the engineers can noodle on a better, cleaner way to make this work?