If you're experiencing excellent signal strength but no connection, there are a few possible explanations.
1. Your device may be out of service. Check to see if there's an outage in your area by visiting your carrier's website or contacting customer support.
2. You may be in a location with limited or no coverage. If you're in a rural area or underground, for example, you may not have any signal at all. But even in urban areas, buildings and other obstacles can block cellular signals. So if you're inside and not near a window, that could be the problem.
3. There could be an issue with your device itself. If you've recently dropped it or gotten it wet, that could be causing the problem. Try restarting your device and see if that helps. If not, you may need to take it to a repair shop or contact your carrier for further troubleshooting assistance.