I suspect it depends on what type of garage, and what the conditions are in there.
Many garages can be mildly damp in the winter months, that would be an unsuitable location in my opinion. However, if clearly dry, maybe even heated .. then it ought to be OK.
Next you need to consider how easy a cable run it is from CSP to where the customer requests the ONT to be sited. Is there permanent available close by ?
The ‘can it be mounted up high’ question makes me wonder if you have selected some weird vaguely inaccessible spot for the ONT.
Lots of questions, but on the day, it’s down to the installer for the yay or nay.
If you really desperately need it somewhere else, you could pay to have it shifted.
One good thing about music, when it hits you feel no pain.