My iPhone can't connect to any modem wifi without Airport Modem

Hello there, SevenK.

The following Knowledge Base article offers up some great steps for troubleshooting various Wi-Fi issues:

iOS: Troubleshooting Wi-Fi networks and connections

http://support.apple.com/kb/ts1398

These initial steps have proven helpful to others:

1. Be sure you're in range of your Wi-Fi router (access point).
2. Tap Settings > Wi-Fi and turn Wi-Fi off and on. If your Wi-Fi setting is dimmed, follow these steps.
3. Confirm that your Wi-Fi router and cable or DSL modem are connected to power, turned on, and connected to the Internet. If not, refer to your network administrator or Internet service provider (ISP) for assistance.
4. Restart your iOS device.
5. Tap Settings > Wi-Fi and locate the Wi-Fi network to which you're connected.
6. Tap and Forget this Network.
7. Try to connect to your desired Wi-Fi network.
   Note: You may need to enter your Wi-Fi password again if your network requires one.
8. Turn your Wi-Fi router off and on2. If your ISP also provides cable or phone service, check with them before attempting this step to avoid interruption of service.
9. Update your device to the latest version of software.
10. Update your Wi-Fi router to the latest firmware2. For AirPort Base Stations, install updates using the AirPort Utility.

Thanks for reaching out to Apple Support Communities.

Cheers,

Pedro

What wi-fi protocol are they using? You can try changing that, or changing the broadcast channel.

I had a problem connecting a new iPhone 5c and my iPad to wifi after the latest upgrade. I was trying to connect while my iMac was runnung and time capsule was on. When I came it to work this morning I decided to try and connect to the wifi on both iPad and Iphone while the mac was off and the power lead was disconected to the time capsule. It connected first time to both devises and is still connected and everything is now running fine. All I have to do now is get my iphone to connect to the network at home. I shall try connecting with no computers running on the network and see if that works. Good Luck.