The safest way to install a stock ROM without a PC is the "dload" method:
If you cannot unlock your bootloader, you can still achieve a "custom ROM" feel through deep system modifications: How to Make the Mate 20 Pro Look Like Stock Android huawei mate 20 pro rom
Custom ROM development for the Mate 20 Pro is hampered by the difficulty of unlocking the bootloader. Huawei Mate 20 Pro - Request a Device - /e/OS community The safest way to install a stock ROM
The story of the Huawei Mate 20 Pro and its software (ROM) is a tale of a device that peaked just as the geopolitical landscape for its manufacturer shifted forever. Launched in late 2018, it was widely hailed as one of the most innovative Android phones ever made, offering a glimpse into a future that was ultimately cut short for international users. The Peak of EMUI Mate 20 Pro arrived, it debuted with The Peak of EMUI Mate 20 Pro arrived,
Fingerprint scanner doesn't work after flashing LineageOS. Solution: Downgrade to EMUI 10 (specifically version 10.1.0.320) via DLOAD, then re-flash the custom ROM. The fingerprint calibration is stored in the persist partition, which custom ROMs sometimes corrupt.