Help article
I'm getting the error "Unable to activate mobile data network" "PDP authentication failed" on my device
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If restarting your device doesn't fix the problem, go to your phone's Settings. Then, under the General tab, choose the Reset option. Select Reset network settings to restore these settings to their defaults. Remember to reconfigure your APN settings afterward. Keep in mind that this action will also erase all saved network information, such as Wi-Fi passwords.