Setup:

Service:

• "Gigabit" cable connection on a Netgear Nighthawk CM1100 DOCSIS 3.1 Modem

Router:

• TP Link Archer C9 ("AC1900") V1.0, setup with above mentioned firmware.
  • I recently updated this device to this firmware; I do not remember the previous firmware version, but my issue has persisted on both firmware versions.

Hardware:

• Windows 7 Computer, dual gigabit LAN, controller driver just lists "Realtek PCIe GBE Family Controller"
  • Various CAT6 ethernet cables I have interchanged for testing.
  • This computer does not have a WiFi antenna/controller.
• Motorola Stylus (cheap-but-modern) Android Smartphone running red velvet. Only mentioning this phone as a comparison tool.

Problem:

• Internet service delivers 350-400 Mbps downstream when connecting my computer directly to my modem. Obviously this isn't actual gigabit speeds, but that's a problem with my modem/ISP and not the focus of this thread.
• When connecting the same computer to this service via the TP-Link router, my downstream tops out at 85 Mbps. This is very slow compared to the direct-to-modem-connection.
• Android smartphone using the 5ghz WiFi network from the TP-link router tops out at 185 Mbps, which i would expect based on it being a wireless connection. However, it's also almost twice as fast as the gigabit wired connection to my computer...
• All settings on TP-Link router are currently at their defaults. I don't know much about these settings so I don't know where to start.

What should I do to try and improve my wired connection speeds between the router and my computer? I can look up any additional information about my setup if necessary.

Thanks.