''Engineering Mode'' is a graphical interpretation and it is more accurate to measure the true signal strength, rather than relying on the bars at the top of your screen.

iPhone:

Please go into settings -> mobile -> mobile data options -> enable 4G / disable 4G (to measure 3G).

Wait until your phone shows 4G or LTE next to the signal bars.

Please go to the keypad as if you are going to make a phone call, dial this number *3001#12345#* press call.

Go to LTE -> Serving Cell Measurement ->

It should show you an RSRP number like -85dbm this is a good signal, -75dbm is a stronger signal, -95dbm is a weaker signal.

Samsung:

Please go into Settings ->More Networks -> Mobile Networks -> Network Mode -> select LTE for 4G/ WCDMA for 3G.

Dial this number *#0011# and your phone will display a ''Service Mode'' page. 

It should show you an RSRP number like -85dbm this is a good signal, -75dbm is a stronger signal, -95dbm is a weaker signal.

Below are the RF measurement performance tables.