My phone have no cellular service
So for an entire day my iPhone 12 Pro Max had no service but for some reason when I plugged it in to charge the service comes up and as soon as I unplugged the phone it’s gone
iPhone 7, iOS 15
You may have a battery that is past its useful life and can't power the phone properly. First, try a forced restart. If a forced restart doesn't fix it, go to Settings > Battery > Battery Health. If its near or below 80% its time to replace the battery.
Try a forced restart.
• Press and quickly release the volume up button
• Press and quickly release the volume down button
• Press and hold the side button.
• When the Apple logo appears, release the button.
Force restart iPhone - Apple Support (IN)
iPhone Battery Replacement.
iPhone Battery Service - Apple Support