Wi-Fi settings and the speed cap under 2.4 Ghz

[Mkerr33]

I just upgrade my fiber to 600MB but for some reason my upload speed averages around 385mbs and download only around 225mbs.  I know on the 2.4ghz signal it looks like my speed is capped at 400mbs, is that something i can change or is it just that i will always get the lower speeds anytime ite uses the 2.4ghz signal?  The 5ghz signal has a 866mbs cap so i assume anytime i use that signal it will give me as much speed as possible.  I don't know much about this stuff, so if i am asking a dumb question please forgive me.  But can you make the device use 5ghz instead of 2.4ghz, or does it automatically choose which one it uses based on the device it is beaming to?  thanks for any help,

Hardware Version	RBR20
Firmware Version	V2.3.5.26
GUI Language Version	V1.0.0.360
Operation Mode	Router
LAN Port
MAC Address	CC:40:D0:4C:11:74
IP Address	192.168.1.1
DHCP Server	On WIRELESS SETTINGS (GENERAL) Region North America   2.4G Channel Auto ( 4 )   2.4G Mode Up to 400 Mbps   5G Channel 36+40+44+48(P)   5G Mode Up to 866.7 Mbps

[FURRYe38]

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.