My iPhone SE has very poor reception, and always shows one bar of service. What do I need to do to have a good communication?

Depending on the build year series, the hardware (antenna?) may have failed.

Three iPhone SE models exist. I've first edition, and the reception varies with

distance from cell tower. When 'local wi-fi' source exists, that may be available

given settings in the iPhone, and may require an OK + Fee from the ISP.

A visit to an Apple store &/or AASP for inspection + testing is recommended.

That is the responsibility of your service provider, not Apple. Perhaps you are too far from their cell phone tower.