My iPhone has recently been stolen, how can I make sure they cannot access my private information?

Mark it as lost. If it comes online, it will not let someone in it.

See this link --> If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

You did the exact thing you should NOT have done and that is remove the phone from your device list. Now the phone is totally accessible to whomever took the phone and there is nothing you nor Apple can do to stop the thief from using the phone and accessing any of the information on the phone.

Change every single password you have, for every single account you have. Had you not removed the phone from your devices, and had you enabled Find My before the phone was stolen, you could have marked the phone as lost or stolen, which would have prevented the situation you are now in.

Yes. Blacklist the device. You were never in danger of losing your phone number. A new sim in a new device and your phone number would be ported to the IMEI on the new device. But make sure no one can EVER use the phone they stole. Blacklist it.

Who are you and what does your comment have to do with the OP's situation? If you have an issue, you need to start your own thread with details about what you need.

marnus128 wrote:

Still can’t log out from that id

OK.....it has to come online before the lost mode works.

The chances are you'll never see this phone again. Definitely contact your cellular provider and ask them to Blacklist the phone.

Still can’t log out from that id

marnus128 wrote:

Still can’t log out from that id

What on earth are you talking about? Your post is meaningless in the context of this discussion. Did you mean to post to a different thread, perhaps?