Please try the following:
1. Ensure you're in a location with strong network coverage. Closed environments like high-speed rail generally provide poor coverage.
2. Remove or replace the protective case if using a metal one. Plastic or leather ones generally enable better network connection.
3. Replace the SIM card to check if the issue is to do with the SIM card.
4. Ensure you're not exceeding your monthly traffic limit, as this may result in traffic restrictions. Contact your carrier for more information.