iOS 11 Field Test Mode permanent signal reading
Previously you were able to replace the bar signal indicator with the signal numbers instead since the numerical reception indicator is more accurate. But, now all iPhones on the newest version of iOS have only one way to view the numerical reception indicator which does not replace the bar signal indicator permanently.

You have to dial the number *3001#12345#*. This will redirect you to the hidden “Field Test Mode” on the iPhone. Then, LTE > Serving Cell Meas > rsrp0.

The value under the rsrp0 will be the numerical measurement of the iPhone cellular signal strength in dBm. You can also read more about this here.