Network strength has two sets of bars

Dual cell signals. On models with Dual SIM, the upper row of bars indicates the signal strength of the line you use for cellular data. The lower row of bars indicates the signal strength of your other line. If there’s no signal, “No Service” appears. To see the status icons with their corresponding cellular plan labels and carrier names, open Control Center.

Learn the meaning of the iPhone status icons - Apple Support

You can also check to have turned off Roaming, and reset the network settings if doesn't work you can try to hard reset the iPhone to see if it's getting sorted

Force restart iPhone - Apple Support