Having the same issue, well sorta. Can't connect to the 5Ghz AP from more than 1 wall away and even then it gives some issues on certain devices where the speed will just tank and latency spikes.
Im actually using an additional router for wireless atm, because the 2.4Ghz AP on the thing also seemed pretty weak when running in dual mode.
I bought that router as I thought it will be much better than the free one. Maybe a setting that someone can direct us to