@ErickLennon2020 

Port negotiation depends on the adapters on both ends and the cable.  Make sure your PC has a gigabit ethernet card and the cable is at least a Cat 5e.  For the bandwidth for 2.4 it depends on the usage.  If you have a lot of devices that are further away from the router use 20 MHz,  more devices closer use 40 MHz.  MU-MIMO is a feature we recommend being enabled.

@ErickLennon2020 

The Channel width on the 5GHz follows the same priciple the further away from the router the lower the width the closer devices will be the bigger the width.  A good way to think about it is a sprayer on a water hose.  The Mist Setting will gett a greater area wet but has far less range that say the stream setting.  The stream setting however is the exact opposite.  Longer distance but less coverage.  

As for the Xbox i do believe its Ethernet port is a gigabit port so it likely comes down to the ethernet cable.  printed on the cable will be either the category (Cat 5, Cat 5e, Cat 6, ect.) or the speed rating 10/100, 10/100/1000.  You need a cat 5e or higher or a rating of 10/100/1000 to get speeds above 100 Mbps.  This is not only true btween the router and the client device like your laptop or xbox but also between the Router and the modem. All devices and cables need to be capable of gigabit speeds to get above 100 Mbps

@VGX 

Go to the router's UI.  Click on Advanced > System Tools > System parameters.  Go to Duplex and see what it is set to.  Make sure its either 1000Mbps Full duplex or Auto Negotiation.  also make sure all ethernet cables Modem to router and Router to client are all Cat5e or higher.  

@shawaiz 

Usually this is only available on routers with gigabit ports.  The Archer C20 has 10/100 ports so the setting is not available on it.  For this concern i would agree.  I'd replace the ethernet cable but also check with your ISP and test the modem.