iPhone dBm signal strength
How to see your true cellular signal strength with the iPhone Field Test app
See the Actual Signal Strength on Your iPhone in iOS
Show actual signal strength (in dBm)

How To Check Signal Strength On an iPhone?

Before beginning, you need to know that this method only works on iPhone models with Intel wireless modem, not on iPhones with Qualcomm wireless modem. That means if you have iPhone 7/7 Plus or later iPhone models, this method will work for you.
In case you have an iPhone 6S or previous models that means your iPhone has Qualcomm wireless modem and thus making this method non-functioning.
Step 1: Launch the Phone app and dial *3001#12345#* and hit the call button.

Step 2: Hitting the Call button will take you to a menu. On that menu, tap on LTE.
Step 3: On the next menu, tap on Serving Cell Meas.

The number adjacent to rsrp0 is the current dBm value.
Now you know the dBm value and you also know how to interpret a dBm value (above). Go ahead, apply the logic and youll know your signals quality.

In the above screenshot, the dBm value is -110 which means the reception is not good.