A6200 suddenly not discovering 5GHz AC network (and not roaming).

I've had the A6200 since launch. It's plugged into my laptop and always stays in my bedroom, where my AirPort Extreme 802.11ac router is located. This router is extended by another AirPort Extreme (802.11n) at the other end of the house.

The laptop always connects to the 802.11ac 5GHz network without any issue at all and consistently downloads at line speed (presently ~180mbps).

Two days ago, very suddenly, it decided that it was no longer going to connect to that 5GHz network, but instead to the Extender's 802.11n network from down the hall (with only 1 signal bar at best). So to clarify, it can technically see 5GHz networks, it just won't see the one that's in the same room as it but will see the one all the way down the hall with almost no signal.

I plugged in an older Netgear 802.11n adapter and it found and connected to the proper network without issue. It roamed when necessary, always switched back and forth based on range, and aside from speed and better range of 802.11ac, it performed exactly as the A6200 WAS performing before two days ago.

Out of everything I've tried, the only thing that allows the A6200 to see the 802.11ac 5GHz network on the proper router is by changing the router's 5GHz channel to manual and setting it to something low like 48. But I never leave it set to manual, it's always on automatic, and everything always tends to work better that way.

Keep in mind that it's only the A6200 having an issue. All cell phones, tablets, other laptops, etc can see and connect to the proper 5GHz network without any issue. I also tested the A6200 on my desktop with a fresh install and it had the exact same issue.

I've tried all typically suggested solutions like disabling security on the routers, etc and none of it made any difference.

Any ideas? All help would be appreciated, this is really driving me crazy.