Bridge Mode with RT-AC68U not working

Here 's the setup

1 ASUS RT-AC68U running as a ROUTER
1 ASUS RT-AC68U running in Bridge Mode

I have connected the bridge mode ASUS successfully on the 5Ghz frequency, but only works as long as I DO NOT have the MAC filtering on.

As soon as I turn MAC filtering, the connection is lost.

Obviously, I have added the Bridge Mode MAC address to the main Router.

Works fine as long as MAC filtering is off.

What gives?

Thanks