https://www.macworld.com/article/3346027/how-to-see-your-true-cellular-signal-strength-with-the-iphone-field-test-app.html
Sometimes you have two bars but can’t load any webpages. Sometimes you have four bars, but your friend with a different phone on the same network has only two. We’ve become reliant on signal bars to tell us how strong our cellular connection is, but in truth, the bars are bullshit.
Those signal bars are almost completely arbitrary. Yes, more bars should mean a stronger signal, but there is no standard scale. Every phone manufacturer gets to decide how much signal strength equals how many bars, and it often varies widely from model to model.
If you really want to know how strong your cell signal is, you need to look at a direct measurement of the signal strength, as measured in decibels (dB). Fortunately, there’s a hidden “Field Test” menu on your iPhone to do just that. It’s not particularly convenient, but it is interesting.
Turn off your Wi-Fi before accessing the Field Test application.
The Field Test menu has changed along with iOS over the years, but this guide should work for anyone on iOS 11 or iOS 12.
First, open the Control Center (or head over to the Settings app) and turn off Wi-Fi.
Then, open the Phone app and enter in the following numbers and symbols, exactly as written (including the stars and pound signs):
3001#12345#*Press the call button, and the Field Test menu will pop up on screen.
Now that you’re in this weird Field Test menu, how do you check your cellular signal strength?
There’s a lot of stuff in here, and it’s not very well labeled. After all, this is not a menu for regular users, but for service technicians.
Start by choosing “LTE” from the first menu.
Choose LTE first.