I have 2 Archer C6 routers that I can't seem to get working with the new Onemesh functionality added in the latest firmware update. Prior to the second C6 I had the following arrangement (picture 1):

I managed to replace the office switch and access point with the second C6 in router mode and currently have it working like picture 2, with the main router connected to the first LAN on the second one, and in management it shows the second router as without internet access (I can't retreive clock info from the internet or setup a wifi schedule):

What I would like to do os connect it like picture 3 and enjoy the benefits of onemesh, but I can't, because despite the second C6 showing up in the main router onemesh settings, every time I click the ADD button I get a " failed"message

Anyone knows if this is a limitation of a 2 router setup, some quirk of the firmware that enabled this feature that was just released, or am I doing something wrong?