SUMMARY
The applet uses binary prefixes (KiB/s) for transmission speeds in the graph showing up/down network traffic over a connection. That means it does not seem to follow the BinaryUnitDialect config option. Likely it is just a matter of replacing some function with some higher-level ones, as other apps do the right thing (e.g., Krusader).

STEPS TO REPRODUCE
1. Set BinaryUnitDialect=2 in [Locale] section of $KDEHOME/.config/kdeglobals (setting to metric/SI instead of IEC)
2. Open network applet
3. Look at a network traffic graph

OBSERVED RESULT
IEC formatting is used instead of metric

EXPECTED RESULT
Metric formatting is used (given existing conventions for network speed being expressed in kbps/Mbps/Gbps <https://en.wikipedia.org/wiki/Data-rate_units>, an alternative might be to use metric multiples of *bits*, as is done in the applet for connection speed in the Details tab)

SOFTWARE/OS VERSIONS
KDE Plasma Version: 5.27.8
KDE Frameworks Version: 5.110.0
Qt Version: 5.15.10

ADDITIONAL INFORMATION
The BinaryUnitDialect config option is described in Bug 57240. This option is for now still without a GUI, but Bug 364321 makes it clear this should change. So apps like this applet should be made ready to follow BinaryUnitDialect (or, because of network speed conventions, use metric multiples of bits).