Also I currently have a custom ROM which was already inside the phone when it was shipped to me
I had this wacky custom ROM too, when the phone came from China. It ain't that easy to just unlock bl and than flash new ROM with it through TWRP. I did it with the instructions i found on 4pda.ru. the main thing is that you need to boot to TWRP right after installation, format data, reboot and enter TWRP again, wipe data/cache/dalvik/system than install new ROM. After installing new ROM BEFORE REBOOT wipe cache/dalvik again. Last reboot. This worked for me