Carrier Unlock for iPhone 8 Plus

I live in Canada and every phone here manufactured 2018 and after is Carrier Unlocked, thus including all iPhone 8 and X series. Today I bought an iPhone 8 Plus from Facebook Marketplace, I checked the IMEI and it wasn't Blacklisted so I bought it but when I got home and put my Telus SIM Card inside it, it appears to be Carrier Locked. I checked settings and this says Sprint SIM Locked, how can I get this unlocked? I contacted the person I bought it from and he said it was his Uncles phone and his Uncle passed away, so he doesn't know how to go about helping me out but he's willing to try. Any suggestions here?