@CCART Any number of things can affect the wire speed. 

Are you using a shielded Cat6 or higher Ethernet cable to the desktop? 

What is the Ethernet speed rating of the Ethernet port on your Deco Router? 

What is the Ethernet speed rating for the Ethernet card in your desktop? 

How is the desktop connecting to the network? 

 

 

I would have guessed that he simply unplugged the working desktop from the old router and stuck that plug into the Deco. Since everything was working before, your only suggestion that makes sense is the rating of the Deco port. Since he is getting 300 mbps, it must be rated greater than 100 mbps. If it is rated for 1 Gbps, then something is wrong with the Deco port.

  @DAServices 

I'm currently using a cat 5 ethernet cable which is what I was using before, however I had already decided that could be the problem so I have a cat 8 cable on order.

Ethernet port on the Deco is rated at gig speed

The Ethernet card speed rating on my desktop is 1000/1000 mbps

The desktop is hardwired to the router

 

The new cable should be here tomorrow, we will see if that helps.