Success!
I have managed to remove the 3BB supplied HG8145V routing-type Optical Network Terminal (which I previously set to bridge mode, so I can use my ASUS RT-AC86U instead) from my 3BB fibre setup and replace it with a HG8310M ONT that I bought off Lazada for 300 baht. Here's a pic of my setup. I've left the HG8145V in the picture so you can see the difference in the size of the devices. I'm keeping it there as a standby for a couple of days so I can easily revert to the old setup, should anything untoward happen.
Replacing the 3BB Huawei HG8145V Router with a Huawei HG8310M Bridging ONT.pdf