I updated my iPhone to 12.4 and now I can not use cellular data

See if these settings instructions help, View or change cellular settings on iPhone - Apple Support

Otherwise if the phone is set with cellular data turned on, then the carrier is the only one that could help. They may need to check to make sure that data is provisioned on your account.

Use a working phone to ask your carrier some questions.

You don't need a new phone. You need a different carrier.

I use Straight Talk from Walmart in my area. Works just fine in my bad reception area.