Ok thank you.You need to have both an unlocked bootloader, and a custom recovery. You have the RMX2151, so the process for unlocking should be a bit easier than for those of us who have the RMX2155. Here's a video:
It shows you how to unlock, and flash a custom recovery onto the Realme 7. I'm going to upload a file with this post: it is the latest stable PBRP release for the Realme 7. Supports all SD Card formats, and is - in my opinion - much better than TWRP. Happy flashing!
But problem with me is I don't have a pc.
I used to use cwm recovery on my old phone.It was a ages ago but looks like it's difficult to install rom nowadays.