[Advice Needed] Home networking for Dummies

[Kalmado]

I feel like I'm not utilizing my equipment to it's best abilities for home networking and to be candid I've done zero research and have never attempted to learn about it. I don't really understand 2.4GHz and 5GHz and how to maximize my devices. Here is what I'm working with.

ISP: Comcast cable performance package (I forget what the DL speed is)
Modem: Motorola Surfboard SB6121
Router: Asus RT-A66U AC 1750
Unmanaged Switch: TP Link 24 port TL-SF1024M
Repeater: 2x Asus RT-N12 N300 3-in-1 Router/AP/Range Extender

I have my main PC, living room TV, PS4, and will have a HTPC hooked up to the switch. Main PC per speetest.net gets 92 Mbps down. If there is wonder about the repeaters, I've had them for years and use them in the upstairs. One is for my wife's desktop. The other is in my work room where I have a PC hooked up to it and will at times have a second PC that I'm working on hooked up to it. Per speedtest.net I get around 5-6 Mbps upstairs.

Other devices include two phones, a netbook, chromecast, and at times guests devices. What I'm wondering is if the repeater speeds are correct seeing as they are N300 devices. If not, or just for good practice, should I be configuring my devices a certain way in my router management?

[synistar]

The repeaters run on the 2.4ghz band based on the specs on ASUS's website. You get good penetration but lower transfer rates. I don't know what is the max transfer rate on those are. Maybe 25Mbps since that was my experience on a 50 down mainline.