prevent downloading of iOS software updates

How to permanently prevent iOS automatic background downloading of system Software Updates to iPhone? I know how to stop Music, Apps, Books & Audiobooks and App Updates but there is no selection to stop updates to the iOS.

These updates burn up my crappy data cap internet (I live in a remote location). Ends up costing me $200 or more per month in internet fees without me knowing till the ransomware demand comes from my criminal provider (Sasktel Fusion) who suspends my internet till I pay.

Sorry for the rant but i hate ransomware.