Welcome to the
Kitz forum.
The Huawei MT992 is a rare beastie and they do not appear to be easily obtained. To date, I have not read any details of one being "forensically" examined and unlocked.
If a locking method, similar to that with the HG612 has been used, then it will require a complete image of the flash contents to be extracted, decompressed, the ACL adjusted to restore access from the Ethernet port before being re-compressed and uploaded back to the device's flash memory.
A simple, preliminary, test that you could do would be to start with the MT992 in total isolation -- no connections to either the DSL or LAN ports -- then press & keep pressed the reset button whilst connecting the power with your other hand. Keep the reset button held for at least 30 second from the time you applied the power. Note any changes that occur with the LEDs. Finally release the reset button and leave the MT992 alone for, say, five minutes. Having allowed the device time to "settle", now connect an Ethernet patch lead to the LAN port and connect the other end to a computer. Configure the computer's LAN port to have an IPv4 address in each of the
private, non-routable, addresses in turn. (At this point the one experiment has been multiplied to three.) I'd start by using 192.168.0.100 as the computer's IPv4 address. Now perform a full
nmap sweep on 192.168.0.0/16 . . . if two addresses are found to be responsive (one will be the computer at 192.168.0.100) then congratulations, you have
probably found the IPv4 address of the boot-loader's firmware update page.
[Or everything could go horribly wrong and the MT992 emits purple smoke . . . never to operate again. ((Q) Why purple? (A) It's the official Openreach colour.)
]
Edit: I see
j0hn's fingers were faster than my paws . . .