The N 7260 is a 2.4 GHz b/g/n adapter. You will only be able to connect to the Nighthawk Mesh on the 2.4 GHz band. It is possible the Nighthawk is using a control channel the N 7260 cannot access.
There are a few additional pieces of information that will be helpful in order to try and help you. First, describe the 2.4 GHz settings on the Nighthawk, but please do not share the SSID secret. Second, please download and run the Intel System Support Utility. Leave the default settings, click Scan, then click Next, and finally click Save. Please attach the saved file and post back. The Intel SSU can be found here: https://downloadcenter.intel.com/download/25293/Intel-System-Support-Utility-for-Windows-
Finally, would it be possible to download and run NetSpot on another wireless PC or laptop, or a mobile phone. NetSpot will report what radio signals it receives from the Nighthawk. Please post back with that information.