Nighthawk AC1900 dropping 5ghz broadcast

> My Nighthawk AC1900 [...]

   "AC1900" is a speed, not a model number.  Ever helpful, Netgear
product packaging emphasizes speeds like "ACxxxx", but that's not the
model identifier.  Look at the product label.

   Firmware version?

   Assuming that it's not a hardware problem, the usual first step would
be to bring the firmware up to date (or load some old, working version).
Then reset the router to its factory default settings, and reconfigure
it.

   Visit http://netgear.com/support , put in your model number, and look
for Documentation.  Get the User Manual.  Look for "Factory Settings".

While you're in the neighborhood, firmware kits should be available

under Downloads.