Wi-Fi settings and the speed cap under 2.4 Ghz

What is the Mfr and model# of the ISP modem the NG router is connected too?

Devices will pick and choose which frequency to connect too based on signal, range and noise. 

What is the size of your home? Sq Ft?
What is the distance between the router and satellite(s)? 30 feet is recommended in between RBR and RBS to begin with depending upon building materials when wirelessly connected. https://kb.netgear.com/000036466/How-far-should-I-place-my-Orbi-satellite-from-my-Orbi-router

Try setting manual channel 1, 6 or 11 on 2.4Ghz and any unused channel on 5Ghz.
Any Wifi Neighbors near by? If so, how many?

Try enabling Beamforming and MIMO(MIMO may or maynot be needed) and WMM. Under Advanced Tab/Advanced Settings/Wireless Settings

Try disabling the following and see:
Armor, Circle, Daisy Chain, Fast Roaming, IPv6 and Set 20/40Mhz Coexistence to 40Mhz only. Save settings and reboot the router and satellite(s).

Are you getting to spec speeds with a wired PC? 

What devices are you using to speed test with? Most current N and AC supporting wifi devcies will cap at around 400-500Mbps depending on what they are designed to support. Something to ask the Mfr about. 

You can help some devices connect to 5Ghz by reducing the power out put on the 2.4Ghz radio from 100% to say 50% and see what this does for you.