SysMon seems to take only a short sample of network traffic each 30 seconds, and if there’s no traffic at sampling time, it shows nothing. - edit: may be or not, but see below! - Just now I started SysMon and started surfing the net and reading a page that causes no network traffic while only open to read it. I scrolled and loaded a new page on this site from time to time. SysMon shows NO cell traffic while reading this page for 15 minutes!
edit: SysMon also shows no traffic while listening to internet radio (permanent traffic)! Neither with nor without VPN. So I suspect there’s a bug in SysMon.