Why mine is 4G, but tonight it has become, and I can't send messages with 3G.

According to the information you provided, if your mobile phone cannot access Unicom's 4G network, it is not excluded that you do not meet the requirements for using Unicom's 4G network.

Using Unicom's 4G network requires three conditions:

1 Use Unicom's SIM card with 4G function (using 4G network requires USIM card);

2 handheld support Unicom 4G network terminal;

Under the coverage of Unicom's 4G network.

It is recommended that you troubleshoot through the above conditions.

If your mobile phone can't send short messages, you can try to restart or change your mobile phone to test whether there are any of the following situations:

1 SMS center number is set incorrectly, please re-enter the correct SMS center number;

2 No signal or insufficient battery power;

3. Because the system is busy during the holiday short message sending peak period, it is suggested that you try again later or avoid the sending peak period;

4 The sending format is incorrect. If you send a short message to PHS users, you need to add the area code before the number;

5 SMS sending time exceeds 24 hours;

6. The mobile phone shields the short message sending function;

7 blacklisted by SMS;

The balance of number 8 is insufficient;

9 SMS Inbox and Outbox are full.

If the above reasons cannot be solved, you can contact the local manual customer service or go to the business hall to confirm whether the SMS function is enabled and hand it over to the staff.