Does ANYONE have any experience with 5GHz Wi-Fi? I'm contemplating buying an adapter AC for my PC but I'm not sure if it would be of much use given the range. My router is approximately 10-12 meters away from the PC. Windows claims that I am receiving 5 bars of signal on 2.4GHz but the speed is LIMITED to 2.5mbs So as an ALTERNATIVE I'm considering an antenna instead.

Just for reference my router is a TP-Link Archer C7. Please no "Why don't you use a cable?" suggestions, if I could run a cable I would.You get 5 bars? Only 2.5 Mbps
That does not sound right.
Let me clarify that statement. I mean that I  get about 20 Mbps on a link with only two or three bars. So it must be that something else is reducing your data rate.  Right now I am Internet via a WI-FI link that goes about 15meters and the includes walls and foliage. Still, I watch  Netflix  on it.

So it must be your have some issue that reduces your speed.
 2.5 Megabytes per second max. On a good day (going on what Windows tells me the transfer rate is).

The signal at the moment is ~-50dBm