It sounds as though you're experiencing some interference when you're upstairs. Possible, at least. But first, just so what below is clear, let me explain some WiFi basics...
Routers all have a 2.4GHz band and most newer ones, all but cheap units, also have a 5GHz band. The router broadcasts both bands simultaneously. 5GHz is usually faster than 2.4GHz but 5GHz doesn't carry as far or penetrate walls as well. Dual-band routers are nice. You get 5GHz speed for devices close enough to the router and 2.4GHz range for farther away.
Your LG G3, being a good, recent model, is capable of using either band. The 'Automatic' setting let's the phone automatically switch to whichever band is available or the best signal. Depending on the signal quality around your house, you might want to change that to use one band or another - or maybe not.
You need to determine the best setup for your situation and configure the router and phone accordingly. If you can't configure the AT&T router yourself, get help from their tech support. They can configure it remotely.
If you have a dual band router, (likely) have them give the 5GHz and 2.4 GHz bands a different SSID. When setting up the 5GHz band set the router to use a channel well over 100. Over 150 if there's a clear channel. For both bands use channels with the least interference and best signal.
To find the best setup I use WiFi Overview 360. An excellent tool, it's the best WiFi analyzer I've found. The free version is probably all you need but the Pro version is cheap. The Google Fiber guys use it to setup WiFi when doing installations.
https://play.google.com/store/apps/details?id=de.android.wifioverviewpro