the reason is
1, the easiest way is to call and check whether your mobile phone is unpaid, if it is unpaid, then you should recharge your phone.
2. Check whether your network icon is normal and whether the traffic is turned on.
3. Check whether your APN access point is normal.
4, you can also restart the phone, and then try to connect to the network.
5. When connecting to WiFi, check whether the WiFi password entered by yourself is correct.