If you have asked Optus and they don't supply it, an engineering student in Adelaide appears to make them on a 3D Printer - https://www.thingiverse.com/thing:3771562 . His details are viewable on the Nortronics link.
eBay also has small adjustable shelves for set top boxes and other small electronic equipment with cable slots that may also be suitable.
Unfortunately, we don't supply wall brackets/mounts. As for the advice you were given regarding outages, I don't think that's correct. If anything, mounting the modem above ground level should positively impact speeds/signal stability.
I would also like a wall bracket. I'm sure that there are lots of other people want one as well.
The modem is designed with some mounting holes - so Sagemcom must be able to supply ??
Can Optus please arrange to make them available - as we all don't have a 3d printer to print the Adelaide Student's design.
I've done a bit of research and I cannot find any information on wall brackets specifically designed for use with the 3864 modem.
We no longer offer the Sagemcom F@st 3864. All Fixed Broadband customers now receive the Sagemcom 5366TN. This is a free standing modem which does not have any mounting holes.