Tip by TitaniumDust
Depending on your configuration, sometimes Windows (and thus Rainmeter) seems to get confused about the Interface= number required for the Measure=NetIn and Measure=NetOut built-in measures for retrieving the amount of data being sent and received over your NIC. (Network Interface Card)
An alternative is to use the Perfmon Plugin. This uses the actual text name of your NIC instead of the number assigned by Windows, and seems to be more reliable for some users.
Using Measure=Plugin with the "Perfmon" plugin as in the examples below, set "PerfMonInstance" to the name of your NIC. To find the name, you can run Perfmon.exe in Windows and press the "Add" button. (the green plus sign above the graph on the right) Select "Network Interface" under "Available Counters" and find your NIC under "Instances of selected objects". Then use the name of your NIC card, EXACTLY as it appears in Perfmon.exe, in the "PerfMonInstance=" setting for the PerfMon plugin in your measure.