Airport Express used for Airplay with Non-Apple Router

The express can work in client mode to any wireless router. It is not necessary to use them as part of the network other than airplay device.. that is their great advantage to anything else on the market.

AirPort Express: How to join an existing Wi-Fi network in client mode - Apple Support

When you use the express this way .. they do not work as extender or repeater.. just a dumb client for which they have all the attributes required.

Express are toy routers.. you should not use them for anything but the end of the chain airplay only devices in client mode.

If the express are running 7.8 firmware go back to 7.6.9 or even 7.6.8 as the newest one is known to be very buggy.

The Spectrum techs are correct.. if you want decent speed over wireless you need better routers.. choices abound and using ethernet is far superior to mesh.. some mesh have ethernet connections as well but you are going to pay a lot of money for a full set. eg Netgear Orbi. It will depend on the size of the house and how much ethernet you are prepared to put in.. !!

Remember ethernet is 100% reliable and fast.. wireless or any other alternative like powerline adapters work well, poorly or somewhere in between and only that great engineering method suck and see.. will tell you the answer.