It is all a bit confusing. AT&T product pages for the AT&T Beam (AC340U) do not appear to say anything about Mac OS X support, but a "Device How-To Center" page does. The latter also mentions Netgear as the manufacturer. At a Netgear site, the following page could be found:
http://kb.netgear.com/app/answers/detail/a_id/23444
I do not know whether or not the web page above (or the problem/procedure described) is at all relevant to your very modem and the network operator in question. I agree with LowLuster, do contact AT&T tech support and ask them to sort things out.
Generally speaking, I do also agree with Linc Davis about USB mobile broadband modems. The need for special driver software often leads to difficulties. A mobile broadband hotspot (3G or 4G), using Wi-Fi for the local wireless connection, is usually a better solution. An added advantage is that the hotspot location can be changed for optimal radio reception from the mobile tower.
Jan