Poor wifi connectivity and slow internet

I have an airport express wifi network connected to a router that has a high speed fibre broadband connection but I get very slow internet and poor wifi connectivity despite multiple airport devices. In some areas unless the device I am using is right next to the airport express its not even detected. Additionally sometimes it's possible to have one device - eg MacBook that connects while another iPhone 6 cannot detect the network at all even though they are next to each other.

There are other networks visible from neighbours and I have chosen channel 6 to avoid these but this doesn't seem to have helped.

Any ideas please?