Firstly, please make sure the inserted SIM card is oriented correctly.
Then check if there is a signal bar or 4G indicator in the upper left corner. If it doesn't appear, it means the SIM card is not being detected.
If the SIM card is detected but there is no network, you need to check the APN information of the card through the operator and fill in the box settings - WIFI - Mobile Network - Access Point Name - click on the top right corner to add.
You may refer to the video below to add APN information.