Unsure if this is the right place, but I have to define my internet speed every time my PC restarts

[Awhni]

Sorry if I posted this in the wrong section...
I have tried resetting the router and modem to no avail.

My Modem is Netgear Nighthawk CM1200

Basically I'm rated for 600 Download/15 Upload 

Under routerlogin, section Dynamic QoS
I disabled Dynamic QoS cause it slows down my speed from testing.

When I try
"Let Speedtest detect my internet speed" (Always under 300 download but 15~20 Upload)" -Incorrect

"I Want to define my internet bandwidth" -I set it at 600 download and 15 Upload -This always fixes the issue for me, This would be the end of the issue but I have to re-define my internet bandwidth through router login every time I restart my PC and I was wondering what can I do to fix this. 

Actual download speed pre-define is around ~20~40
Post Define Speed goes to 60~80
It's really just a minor inconvenience and not a big deal but I don't want to be doing this everyday if I really don't need to.
If this is an IPS issue, what should I ask to fix this issue. Thank you.

I never really had to deal with router and modem and stuff so sorry if I'm a bit slow on understanding what I have to do to help my situation. Thank you so much for any help that can be given.

[plemans]

What firmware is on the R8000?
if you're speeds are >250mbps, qos isn't recommended. 

I'd try disabling it and rebooting the device. 

[michaelkenward]

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@Awhni wrote:
...I have to re-define my internet bandwidth through router login every time I restart my PC

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What prompts you to do that?

What message appears on the PC?