AC1900 Doesn't Connect to 5ghz

Hi, my AC1900 randomly stopped connecting to my 5ghz Uverse router. The 2ghz works perfectly and I've reset the router and extender several times. After every full re-set the 5ghz works for some period of time but routinely stops working. I'm able to connect to the regular 5ghz so I know the router itself is working but the extender is unable to pick it up and extend it. As the 5ghz is mich more powerful, this is affecting many of my applications. Any tips? Thanks!