You can run sysdiagnose on your phone (press volume up, volume down, and lock button) all together for around a second. You will feel the phone vibrate. Wait a couple of minutes and then go to Settings -> Privacy and Security -> Analytics and Improvements -> Analytics Data.
You will then scroll down and look for a file titled: sysdiagnose_XXXXXXXX
You will click on that file and then click the arrow in the upper right hand corner and scroll down and click "Save to Files"
Once it is saved you will go into your files and click the file to unpack it. Within the folder that is unpacked you will click on the folder "ioreg" and then click the file "IODeviceTree". You will then use the magnifying search glass in the bottom right hand corner to search "raw-panel". It will then give you a serial number.
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