There are two types of firmware available online:
A: Only if the custom firmware retains Widevine L1 certification. Most modified firmwares lose L1, downgrading to L3 (480p max). Stock ISP firmware usually has L1. Zte Zxv10 B760d Firmware
Pro tip: Before attempting any firmware operation, disconnect the box from the internet to prevent auto-updates from your ISP that could overwrite your custom flash. There are two types of firmware available online:
Safe sources (verified):
Avoid: