RT-AC68U in Repeater mode with RT-AC5300 not working

I have the RT-AC5300 (FW: Asus 380.7743), I just purchased the RT-AC68U (FW: Asus 384.10007) to extend the RT-5300 wireless network by using it in Repeater mode, I've tried all the possible combinations, extend the 2.4Ghz or the 5Ghz wireless networks. Nonetheless it has been impossible to make it work, not even once, I always get the messages (screen shot below as well):

"Parent AP status disconnected"
"Unable to connect to parent AP"

Any idea how to get this up and running, I followed...

RT-AC68U in Repeater mode with RT-AC5300 not working