Unable to access certain websites using Safari on iPhone

That message means that Safari is protecting you from something shady going on with your network connection. It is possible that someone is impersonating those websites or portions of them.

Some things to consider are:

1. Do you get that same warning if you test on cellular data, and also test from multiple WiFi networks? If the result is the same on all networks, then it's likely something installed on your iPhone. If the problem only occurs on a specific network, then avoid using that network because the operator may be attempting to impersonate websites or inject content.
2. Is your iPhone "managed" by a company or school in any way? If so, the management software could be attempting to snoop on those websites content.
3. Have you installed any VPN software, security software, or testflight software on the phone? If so, go to the Profiles page in settings and remove all of the profiles and associated apps before testing again.
4. Lastly, check to see if you have any extensions installed in Safari (Get extensions to customize Safari on iPhone - Apple Support). If so, remove the extensions and then test again.