Extending a wireless network from an Arris TG3482G Router

I just moved in to a new house and got new Xfinity service, the 600MBPS Down 15MBPS up version.

 

The Arris TG3482G router Xfinity provided (rented I guess) is located in the ground floor office but reception downstairs in my studio is really slow when using speedtest. It goes from around 570MBPS in the upstairs office to less than 100MBPS downstairs even though it is only about 25 feet away with no crazy solid walls or anything in the way of the signal.

 

Initially I thought I could use all of my Apple Airport stuff that I still have which consists of:

1 x Airport Extreme Base Station (2017 model)

2 x Airport Express

1 x Airport Time Capsule

 

I turned the Xfinity Arris TG3482G router to bridge mode to turn off the wireless capability of it, plugged in the Airport Extreme Base Station via Cat5, and placed the 2 Airport Expresses in different parts of the house to try to extend the range down to the downstairs studio.

 

Everything worked correctly in terms of showing up as expected but the speed decreased to less than 100MBPS when standing right next to the Airport Extreme Base Station upstairs and to less than 10MBPS downstairs even though I was connected through the Airport Expresses right beside my Macbook Pro.

 

In other words, it was a complete fail, so I resorted to turning the wireless capability back on on the Xfinity Arris TG3482G router and ditching the Airport devices for now.

 

So........My question is: what is a good wireless extender for the Arris TG3482G that will give me coverage in the other parts of my house with a similar speed to what I get when I'm standing in the office next to the Arris? (i.e around 500MBPS +).

 

Any thoughts on why the Apple Airport stuff behaved so poorly is definitely welcomed (as in maybe I didn't set it up correctly, but I really think I did) but I'm really just in the market to buy a more modern (non Apple) wireless extender and want to know what my options are.

 

Thanks in advance for any help members here can give.