Could you explain it simpler? I'm not that good at tech
An example: If you live in a house, your house' address is unique, and whenever you send a mail, it returns back to you, you have it. You then know which room to deliver your mail.
If you live in an apartment building, there's a doorman who keeps track of the outgoing mail, so the doorman delivers the mail to the correct apartment when it comes back. But the doorman cannot deliver it to any particular room, since this info is not present in the outgoing mail, and doorman has no idea your apartment has multiple rooms.
When you subscribe to a "standard" ISP service, youi're living in a house - you are given public IP address. When you subscribe to mobile ISP, you're living in an a room in an apartment, and your apartment is not directly accessible from outside. That's why no packets reach your router.