The command line can be used to ping with specific Maximum Transmission Unit (MTU) sizes and set a flag to indicate when fragmentation occurs (meaning the MTU at this point is too high). This method helps identify the ceiling of the LAN connection as well as the WAN packets coming from the carrier. It may be a bit tedious, but it is effective. See MTU/MSS Design Considerations for more information.
Common MTU Values:
LAN connection to LAN client – 1,500
WiMAX wireless WAN connection – 1,400
2G/3G wireless WAN connection – 1,350–1,500 (carrier dependent)
LTE/5G wireless WAN connection – 1,428
Cable/DOCSIS modem wired WAN connection – 1,500
DSL modem wired WAN connection – 1,492