Router Firmware and DNS (SKY UK)

Good morning, I am from the UK and I my ISP is Sky. I wanted to change my router to an open-source firmware, but I don’t know which one to choose. Also I tried to change my DNS to Cloudflare and when I did a DNS leak test, it kept on showing my ISP’s IP address.
My sky router model: SR203

Any Help is Appreciated,
Thank You.