iPhone 8 is showing no network provider

You must subscribe to a network provider. You do not get a list of providers. You either buy a prepaid SIM card or you contact a cellular network provider in your area and create an account with them. Once you do this they will give you instructions on how to connect your iPhone to their network.

No, what is the exact message? And what country are you in?

Perhaps you should consider posting in your native language.

If you lose a Wi-Fi connection any app will switch to cellular data.

That should not trigger a security issue, but it could result in unexpected usage charges.