I've been experiencing this WiFi issue recently. I cannot say if the cause was upgrading to iOS 7, because I noticed the same problem happening on iPhone 5 (iOS 7) and iPhone 3GS (iOS 6.1.3):
- losing WiFi connection frequently
- "weak" WiFi signal as if I was far away from the router
- sometimes cannot even see the WiFi network (even in the same room of the wireless broadband modem/router)
- constantly on 3G instead of WiFi before unlocking
- WiFi connection possible after disabling/enabling WiFi or enabling/disabling Airplane mode; in any case after few minutes, both phones were on on 3G again
Other PCs and MACs had no connectivity problem at all. I'm using a D-Link DSL-2740B.
These steps didn't help:
- router: change from WPA2 to WPA
- router: disable Wireless Filter / MAC filter
- router: save settings - hardware reset to factory defaults - restore settings
- router: reboot
- iPhone: reset network settings
- iPhone: hard-reset (Power+Home button)
Solved my problem (but it could be just my own case) by:
- iPhone: remove saved WiFi network
- router: hardware reset to factory defaults (using the button on the back of the router)
- router: power off then power on
- router: manually re-enter all settings (didn't enable Wireless Filter yet)
- iPhone: join to WiFi network (same SSID and WPA Key as before)
Now it works perfectly as it is suppose to. After 3 days I didn't experience any WiFi issues both on the iPhone5 and the 3GS. Hope this can help.