Thank you for your prompt response !
No I do not use MT300A as a second wifi … it is only connected via ethernet (on Apple Airport main router) to use OpenVPN only for the Apple TV. My other devices do not need to be on the vpn.
OpenVPN on MT300A works fine, all I just want is that the other devices on the network 192.168.1.* see the Apple TV IP address was given by the MT300A…
(Sorry for my english, I hope it is more clear…