Without ROOTING your phone, you can use the ping command from a Windows / Mac / Unix system.
For Windows, try the following:
ping -l 1473 -f 10.68.34.75
Adjust the payload using the -l command line option. When you reach a higher limit, you will see this message (French) and you will find the MTU size:
Le paquet doit être fragmenté mais paramétré DF.
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