PROXY AUTHENTIFICATION REQUIRED

If you're encountering a "Proxy Authentication Required" popup on your iPhone, it means that your device is attempting to connect to the internet through a proxy server, and the proxy server requires authentication before granting access. Here are a few steps you can take to resolve the issue:

1. Go to the "Settings" app on your iPhone and tap on "Wi-Fi." Make sure you are connected to the correct Wi-Fi network and tap on the (i) icon next to the network name.
2. On the Wi-Fi settings page, tap on the "Forget This Network" option. Confirm your selection and then reconnect to the network by entering the password.
3. If forgetting the network didn't work, you can try resetting your iPhone's network settings. Reset Network Settings: Only network settings are removed.
   1. When you reset network settings, previously used networks and VPN settings that weren’t installed by a configuration profile or mobile device management (MDM) are removed. Wi-Fi is turned off and then back on, disconnecting you from any network you’re on. The Wi-Fi and Ask to Join Networks settings remain turned on.
   2. To remove VPN settings installed by a configuration profile, go to Settings > General > Profiles & Device Management, select the configuration profile, then tap Remove Profile. This also removes other settings and accounts provided by the profile. See Install or remove configuration profiles on iPhone in this guide.
   3. To remove network settings installed by MDM, go to Settings > General > Profiles & Device Management, select the management, then tap Remove Management. This also removes other settings and certificates provided by MDM. See “Mobile device management (MDM)” in the iOS Deployment Reference.
   4. Reset your Network Settings. Tap Settings > General > Reset > Reset Network Settings. This also resets Wi-Fi networks and passwords, cellular settings, and VPN and APN settings that you've used before.
4. If you intentionally set up a proxy server on your iPhone, you can try disabling it to see if it resolves the issue. Go to "Settings" > "Wi-Fi" and tap on the (i) icon next to the network name. Scroll down and tap on "Configure Proxy." Make sure the "Auto" option is selected or disable any manual proxy configurations.
5. If you're connecting to a network at your workplace or school, it's possible that the proxy server settings have changed or require additional authentication. Contact the network administrator or IT department for assistance.

It would be better if you can please re-post with a screenshot, in order to understand the phenomenon better. Images/pictures speak louder than words. 

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With the limited information that you have provided, It is hard to even guess the problems faced by you. 

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Please re-post with detail so that someone else can help.