4ch1ll I've been using this workaround for the last six months and I was quite happy with it, but after the latest update, the data usage settings shortcut was removed and now I'm stuck with the same problem again, it's still very unconvenient to always switch to the owner profile. Is there another simple workaround?
Just found it and realized @Novaliss was correct all along, not sure what I did wrong back then.
Install ActivityManager. Launch it and search for "Settings" in the app. Tap the result showing com.android.settings and then search for "mobilenetworklist". It should return one activity. If you tap it to launch it, you'll get a page normally only visible in the owner profile settings under "Network & Internet > SIMs". At the bottom is a mobile data toggle. Back in ActivityManager, hold the activity and select "Create shortcut". You can add that to your homescreen for quick access.