I'm running dd-wrt on my Linksys 1900ACS V2.
My shiny new OnePlus 5 couldn't see the 5GHz network. This has been an issue on work's wifi too.
I discovered under each of the 2.4 and 5GHz network advanced settings that my "Regulatory Domain" was set to the United States. I flipped this to Ireland and Voila! my phone now happily sees and uses the 5GHz frequency.