Greetings chilli1111,
It sounds like you are having issues deregistering your iMessage. We know how important this is for you, and we will help get you to the right place to find a resolution.
We see you already check out this link: Deregister and Turn Off iMessage - Apple Support
To double-check, did you use these exact steps?
"Deregister iMessage online
If you don't have access to your iPhone, you can deregister iMessage online. After you complete deregistration, your phone number is removed from iMessage services. You should be able to receive text messages right away but it might take a few hours for some Apple devices to recognize that you're not using iMessage when they send you a message.
If someone sends an iMessage to your Apple ID, you'll get it on your other Apple devices. If someone sends a text to your phone number, you'll get the message as an SMS/MMS only on your phone."
If you followed that process and need continued assistance, then please reach out to the Apple support team via this link: Get Support.
Cheers!