How to change the signal strength bar to dBm numbers

I've been using this tip to display signal strength on my iPhones for years but discovered that updating to iOS 11 not only switched the information back to bars but the former system (which also includes turning off WiFi and disabling LTE) no longer works. Specifically, after holding down the power button, then releasing it and holding down the home button until Field Test mode quits and you return to your home screen, Field Test mode doesn't quit that way. You'll see Field Test mode window until you release the home button, then click it again.

TL/DR version; anyone know how to get the signal strength to be displayed as dBm numbers rather than bars in iOS on an iPhone 5S?

Dial the following:

*3001#12345#*

Does anyone know how to get this on an iPad with Data SIM ? ie. can't "call xxxx" to initiate.

Thanks