Hi, please confirm the WAN negotiation speed between the AX50 and your ISP modem, is it negotiated at 100Mbps or 1000Mbps? Please try the below suggestions and get back to the community with the details if it cannot be solved, thanks.
1. Log in to the web UI of the Archer AX50 and go to the Basic-Network map page, click the Archer AX50 image/icon and scroll down this page, you will see the Ethernet negotiation speed of the LAN ports, show us the screenshot:
2. Go to Advanced-system tools-system parameters to edit the duplex, change it to 1000Mbps full-duplex if you have a gigabit line, then reboot the router.
3. After that, try to test the negotiation speed on your PC again. It is strongly suggested changing another cat5e or cat6 cable.
4. It is also suggested changing another wired device with gigabit Ethernet ports to do a test.
5. What is the length of the Ethernet cable between the AX50 and your PC, in other words, what is the distance between them? Is it the same cable when you are testing the speed with your modem directly?
If none of the above works, please tell us the brand and model number of the gigabit Ethernet adapter on that PC, we will check and investigate further. Thanks.