Your card theoretically supports 300mbps, if you're not getting at least the 100mbps (or higher) there's some issue elsewhere. This can be related to distance, interference, the number of people connected to your network, the router is poor quality etc.
If it is interference, then upgrading to 5GHz would definitely fix the problem. I assume this is what the tech might have been referring to.
Also, do note that while you might be paying for 100mbps, your ISP might not be giving you that speed. Pretty much all ISPs have a statement in their contract stating that the advertised speeds are only the maximum of what your plan supports, it doesn't mean you're guaranteed to consistently receive that speed. This is why I suggested doing a test with a direct connection to the router, if you get the 100mbps that way, then we know it's simply a problem with the wifi signal.
