Check the packaging if your stick really supports 5Ghz. I have the same adapter and mine is only for 2.4ghz

TL-WN823N does only supports 2.4GHz not 5GHz network.

Of course, you can't connect to 5GHz with it. In your case, you need a dual band wireless adapter.

Cheers!

Yeah, so I already went back and bought another - now the dual-band AC600.

Well, on the package of the TL-WN823N it says for the supported IEEE standards: b, g, n
If you look at the russian description, there is something with "2,4" - most likely saying, that only 2,4 GHz is supported (as it obviously is). For the rest of the languages, there wasn't anything mentioned about the supported band - but apart from this, this stick obviously does NOT support 802.11n.
So what the fuck? Not sure, which retard is responsible for that really good package design, but if TP-Link wouldn't be the cheapest (in this case for a simple 5 GHz connection), I wouldn't have bought the new AC600.
b, g, n b, g, n b, g, n