For a while now it has not been possible to know exactly what your network device ID will be, it used to be the case that it was nearly always eth0 or if you had more than 1 card it would be eth1 or eth2 etc, wireless cards were always wlan0 (again if you have more than 1 the number would increase). This made it very simple to know the network devices ID for using with many commands (tcpdump, netstat and many more) but now the protocol for defining the device ID has changed and we can no longer assume what they will be without knowing some very specific system details.
To overcome this we need an easy way to find the ID’s and that is using this single command
ip link show
this will list all your network devices that are currently available on your system. You can safely ignore lo as this is the loopback device used for local services, what we would be interested in are any that are something like enp3s0 or wls3 but they can have quite a different look and could contain a lot more characters than this, but they will all be listed for you to see and you could then use them with commands that need them (or within your conky).