Thanks for the information. You have several issues here.
The first is that the A1088 AirPort Express is a much older version of the product....8+ years and probably 10+.....so it has no setting to "extend a wireless network", which is a feature that Apple introduced on later models in 2008.
Next, even if you had a newer version of the Express, it is not compatible with other brands of routers for the "extend" feature. The Express will only extend a wireless signal that is being provided by another Apple AirPort router.
We asked what make and model of router you were using, but did not get an answer on that one, so we will guess that you have a 3rd party router.
Finally, the iPad and iPhone will work with newer A1264 and A1392 versions of the Express, but they will not be able to configure the older model that you have, which was discontinued in 2007.
It might be possible to set up the Express that you have, but it would require a permanent, wired Ethernet cable connection to your main router.....and.....you will need an older Mac running the Leopard or Snow Leopard operating system to be able to configure the older Express model. A PC with AirPort Utility installed on the device would work as well.