Airport Express

If you are planning to "extend" the signal using wireless only, the AirPort Express can only do this if you also have an Apple router as your "main" router.

If you are trying to "extend" the signal from a 3rd party product using wireless only, the AirPort Express would almost certainly not be compatible for that purpose.

The workaround would be a wired Ethernet connection or Ethernet powerline adapters to establish a wired connection between the AirPort Express and your non-Apple router.

In addition to Bob's excellent advice, the Express (if extending wirelessly) must be positioned about half way between the router and the place you want to extend to.