Your local technician will most probably know it but not allowed to share it. I know this because when I used to be live in another country, they used to work with their own plug n play routers and you could only run your own router if you got the VLAN ID from your local technician and the ID was unique depending on what area you were in.
If someone really want to get the ONT in bridge mode...Considering VLAN tag has 12 bits...Just try 4094 combinations and you will get the result.
Perhaps,it would be below 200 for smaller number of connections.
But you can still bruteforce...😂
Just keep trying different VLAN ID's till you get the right one. They specify a range I think currently between 1-4092, so just keep entering and trying it until you get the bridge mode tick on the home page.