If not done already, make the Developer options visible by tapping on the Build number in Settings - About phone a few times until you are being declared a developer.
Then disable Settings - System - Advanced - Developer options - Mobile data always active.
The description of this switch is “Always keep mobile data active, even when Wi-Fi is active (for fast network switching)”.
Sounds promising to me, but please report whether it helps with your networking issue, I just used it to work around a battery drain.