Grabbed the config.bin off my ISP-provided ZTE F670L, decrypted it in about 5 minutes. The "encryption" key is just the serial number base concatenated with the byte-reversed MAC address... both printed on the sticker.
Inside: GPON credentials, TR-069 ACS config, VoIP SIP keys, and a hidden super admin account with full privileges. The password was in HaveIBeenPwned.
Used zte-config-utility. Steps are in the gist.