A couple of days ago, I noticed that only few apps work when I turn on the cellular data. For the rest of the apps, when I turn on the cellular data for them they just turn back off when I exit Settings. This is really inconvenient and I’m unable to

Is this your corporate iPhone? Do you have any profile installed by your corporate / organisation / enterprise / School Management then please contact the system administrator of your corporate / organisation / enterprise / School Management

If you have installed a profile on your own, remove it --> Install or remove configuration profiles on iPhone - Apple ...

Contact your Carrier (Cellular Service Provider) The agency who issued the SIM to you if they have installed anything through their carrier settings --> Update your carrier settings on your iPhone or iPad - Apple Support

Try --> Check the cellular data usage on your iPhone and iPad ...