Wi-Fi signals are affected by your surroundings, the state of your iPhone and router, and the wireless connection between them. You can do a few things to optimize all three for stronger iPhone Wi-Fi signals. If Wi-Fi issues persist at your home or office, update your router's firmware or upgrade altogether to a better-performing unit.
Location and Environment
The closer you are to a Wi-Fi router, the stronger your iPhone's Wi-Fi signal. You can easily figure out a home router's location, but in a public hot spot, it may not be so easy. Ask an attendant where it is or observe your Wi-Fi wave bars as you walk in different directions. Move to within 115 feet of the router. Avoid thick wall obstructions and electromagnetic interference by baby monitors, cordless phones and similar items.
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IPhone and Router Conditions
IPhone or router fatigue can weaken your Wi-Fi signal considerably. Reboot your router or request the same at a public Wi-Fi hot spot, if possible. In any case, restart your iPhone also. Then, tap "Settings," followed by "Wi-Fi" and toggle the Wi-Fi switch off and back on.
Try refreshing your iPhone's Wi-Fi network connection. Tap "Settings | Wi-Fi | [Network Name] | Renew Lease | Renew." Alternatively, touch "Settings | Wi-Fi | [Network Name] | Forget This Network" and then rejoin the forgotten network. You can also rejoin from a completely clean slate -- tap "Settings | About | Reset | Reset Network Settings." You need the password, if any, to rejoin a network, so have it handy.