I have a house with a cellar, two upper floors and an attic. I operate one Archer C6 in router mode in cellar and two Archer C6 for both upper floors configured in access point mode. That works just fine. I have seamless connection when I go up and down. It's always "roaming" seamlessly.

Now, I want to install the wall-mount EAP230 in the attic to operate it just as Archer C6's. But I can't get it work properly in access point mode. When I install it, I only get the option to configure a new Wifi network. I configure it with the same SSID and password but it doesn't "co-operate" with other C6s. I doesn't roam properly as it works with both C6s in the both upper floors. Either I can connect to the wifi network in both upper floors (which are working fine together) or to the EAP230 in the attic. When I go up and down, I lose the connection and need to reconnect.

What am I doing wrong? Is it possible at all that both product lines are working together as access points???

See the installation plan below: