How can I fix the cellular modem

Try a forced restart If your iPhone, iPad, or iPod touch won't turn on or is frozen - Apple Support

If that doesn't help then try "Reset Network Settings" If your iPhone, iPad, or iPod touch won’t connect to a Wi-Fi network - Apple Support

You could see if your device requires a Carrier Update Update your carrier settings on your iPhone or iPad - Apple Support

The last link I posted talks you through how to check and if one is available, but basically you go into Settings/General/About, and if an update is available it will prompt you to install.