Huawei stopped providing bootloader unlock codes around 2018. The NMOL31 likely has a locked bootloader, making custom ROM installation impossible on stock firmware.
Huawei NMO-L31 Custom ROM Verified: The Ultimate Upgrade Guide huawei nmol31 custom rom verified
A "verified" custom ROM is useless if you skip these steps. Huawei stopped providing bootloader unlock codes around 2018