Check whether the 4G service is not enabled and whether the mobile data and 4G switch are turned on. You can enter the settings and set the network to 4G/3G/2G mode.
Or your place has 2G network coverage, or the network signal is not very good, which has nothing to do with the mobile phone, so you can only use 2G network.
See if the mobile phone card is consistent with the operator. Generally speaking, the mobile customized version only supports mobile 4G networks, and will not support Unicom's 4G networks.
Think about changing the set meal. When the traffic of the 4G package runs out, the 4G network will not be used.
Put the card on other mobile phones, if any. It is recommended to try to replace the dedicated mobile phone card in the business hall.
Input settings-> Mobile network->; Access point name, click the menu button to reset to the default settings.
If it still can't be solved, it is recommended to take the purchase invoice to the maintenance outlets for inspection. In order to avoid file loss, be sure to back up the data first.