deregister imessage

I was following the website about how to deregister iMessage. And here the website: https://selfsolve.apple.com/deregister-imessage/

I tried to turn iMessage and Facetime off and waited for a few days, but I still have trouble receiving a group message.

Then I tried to put my phone number into the website to pretend that I do not have my iPhone anymore, even though I still have it, but I do not use it anymore.

However, these two ways are not working. I still cannot receive anything from a specific group chat. I wonder, is it because I change to an android phone, but all other people from the group are using iPhones.

Is this mean anyone in that group chat needs to change their imessage to text message?

How can I solve this issue?