First the obvious: check that the Internet Gate wireless access point is enabled. The AIR LED on its front panel should be lit. If not, enable wireless on the wireless page.
Avoid using channel 11, as it is not allowed in several countries. Even if you have purchased a PC in Europe its wireless card might be configured to US disabling channel 11.
iPhone, iPod, iPad, Android mobiles and other clients might have problem connecting and/or achieving stable connections if Wireless Mode is set to “802.11B/G” on the wireless page. Select 802.11G only instead.
To increase wireless throughput and connection stability always select Wireless Mode: 802.11G only on the wireless page, unless you do have some old legacy clients with only 802.11B capability.