Extending Asus router

I have an Asus AC68U but it does not reach parts of my house. I had a 1st gen airport express, a time capsule (latest version), a 5 year old time capsule and an AirPort Extreme (old but not super old, kind of looks like the flat time capsules) And I wanted to set something up so that it would reach the whole house.

I Used to, a while back, have the old time capsule extend the network for my previous router, which worked fine, but it isn't working with this current one. I'm able to set it up, but then after appoint the settings, it just flashes orange and says it can't find the network it's meant to be extending. I tried with all of the airports I had, but they all did the same. Even when they are literally 30cm away from the router with nothing between them.

I emporarily have set up something that works a bit, which is to have the 1st gen express join the network (it can join but not extend), and then to have the new time capsule connected to that through Ethernet, making it's own wifi network. However it's slow, frequently needs restarting because it drops out or something.

I've just been using default settings on the Asus router and the airports.

Any help with setting this up properly would be much appreciated.