The mobile phone display service is not available.

Try to restart it and see if there is a signal problem.

If not, check whether the sim card is in good contact. You can insert the card slot.

Take out the card and use another mobile phone to see if this card is broken. If it is broken, change the sim card (free of charge).

Try to put a good sim card in.

If it doesn't work, it needs repairing.