Sometimes a simple power cycle of the router can correct the issue that you are experiencing. However, the issue could be a little more complex. Wi-Fi routers need adjustments to settings periodically for optimized performance with the constant progression of wireless device technology. Below, I have included two articles. The first article will cover basic Wi-Fi troubleshooting regarding iOS device connection. The second article cover the recommended settings for current Wi-Fi access points.
Also, I see you have an AirPort Express. Make sure to check for updates to the firmware.