VA-ANDY, welcome to the forum!
I'm an insurance adjuster so fortunately I have some unique insight into this. I haven't actually examined how the CX-9 BSM modules are fastened to the back side of the bumper cover, but if it's like most vehicles, it may be secured via a plastic bracket. Most vehicles I have seen have a plastic bracket, which is integral with the bumper cover, and you cannot purchase the bracket, you have to buy the whole bumper. What I usually see is when the bumper gets hit, it flexes inward and the bsm bracket becomes partially un-sealed from the bumper.
They way they are usually mounted is simply via some sort of plastic welding or a real strong 2 part epoxy at the factory. Basically it's either melted to the bumper or glued on.
How to fix? JB Weld would be the easiest solution. You can buy it at walmart, home depot, lowes, etc. Its 2 tubes of paste that you mix together on a piece of carboard or similar material, and then apply as needed.
It's very strong stuff, supposedly it can used on engine block cracks. (urban legend)
Just be careful if you go about it this way that you don't get any on the actual computer module and I wouldn't goop it up too thick. You may want to sand the area down and remove the factory goop before applying any new material. Added thickness may reduce the effectiveness of the module. Try to match the factory thickness of whatever is there.
If JB Weld is too thick, which it usally is pretty thick, you could also buy a 2 part epoxy at home depot/lowes, as epoxy is usually a little bit thinner than JB weld and is also pretty strong.
OR, you could ask a body shop to do this for you, but since it is a "safety" system, they might want nothing to do with it out of liability reasons, in case you wreck or something.
Hope this helps.
Not sure why the dealer is telling you to buy a new module if it is just the bracket....unless the bracket is actually replaceable. Not sure.