My Roku was detecting my 2.4GHz SSID fine, but not my 5GHz SSID. Roku support talks about 5GHz not supporting DFS channels. This was not the problem for me because my router doesn't use DFS settings that I know of. What was the problem was that it doesn't recognize 80MHz bandwidth 5GHz channels, I reduced the bandwidth to 40MHz and pop, the Roku autodetected it and everything worked fine from then on. Can't believe support doesn't mention this, that I could find.
Reducing 80Mhz to 40Mhz channel bandwidth also resolved my 5GHz connection on my streaming stick+ to ASUS RT-AC66R router, major oversight on the design side that will cause may customers issues, hope its software.
I had the same problem. Figured roku does not support 80mhz only 20 and 40mhz.