@Sunshine I've not found a way to definitively check the negotiation speeds, but it's the only explanation I could logically assume, based on the fact that it caps at exactly 100Mbps.

The version of my router is definitely EU, but any later versions than the one I currently have installed (e.g. Archer C2(EU)_V1_160706 or Archer C2(EU)_V1_170207), when attempting to upgrade to these versions, I get the error code 4503/region mismatch, as stated in the last paragraph of my post.

At the moment, my PC is connected to the Archer C2 via an unmanaged D-link 5-port gigabit switch. It is not that my PC connection is not at gigabit, since transfers in-between PCs on the same internal network is at gigabit. It is the line speed between the ISP modem (Archer C2) and the Fibre ONT/CPE (Raycore).

Order of events:

- I realise any device on my network can only achieve maximum 100Mbps from the internet

- Restart the router

- back on 200Mbps for an indeterminable amount of time

- after a couple of days, again I realise that the speed has dropped to half

- Restart the router

rinse and repeat

The only questionable occurrence I can find on the log files is that, at some point, the ISP modem re-requests a WAN IP from the ISP, which I am not sure why since the network remained active throughout. As below:

2023-10-08 11:37:41 [5] DHCPC: Send REQUEST to server 2.2.2.2 with request ip 1.1.1.1
2023-10-08 11:37:41 [5] DHCPC: Recv ACK from server 2.2.2.2 with ip 1.1.1.1 lease time 86400

I replaced my public IP with 1.1.1.1, and what I assume is my ISP with 2.2.2.2

When I do a release/renew request from the Dynamic IP WAN interface, the log shows the following (after which the line is still on 100Mbps):
 

6    2023-10-08 23:11:58    DHCPC    Information    Unicasting a release of 1.1.1.1 to 2.2.2.2
5    2023-10-08 23:12:01    DHCPC    Notice    Send DISCOVER with request ip 0.0.0.0 and unicast flag 0
4    2023-10-08 23:12:01    DHCPC    Notice    Recv OFFER from server 2.2.2.2 with ip 1.1.1.1
3    2023-10-08 23:12:01    DHCPC    Notice    Send REQUEST to server 2.2.2.2 with request ip 1.1.1.1
2    2023-10-08 23:12:02    DHCPC    Notice    Recv ACK from server 2.2.2.2 with ip 1.1.1.1 lease time 86400
1    2023-10-08 23:12:02    DHCPC    Notice    Recv DNS server address 8.8.8.8,8.8.4.4

Note that the above instances are the only ones labelled as DHCPC, the local IP Allocation requests to the DHCP server are all DHCPD categorized.

Quick Update: For some reason, after re-trying the firmware update for Archer_C2v1_0.9.1_4.2_up_boot(160706)_2016-07-06_11.05.02, it actually took it!
Time will now tell whether this had any impact on the internet speed.

It seems that the router log is entry-limited, so I'll have to check back daily and copy the logs until the speed-issue re-occurs.

  @Sunshine, just an update, my router has been running for days on end (with the occasional loss of internet from the ISP necessitating a restart) without dropping the negotiated line speed at all, this makes me so happy! I'm currently on 9 days and 7 hours with no issues.

Thank you again for assisting me. I've left the Firmware on the currently installed version. (If it ain't broke, don't fix it! )
If I run into an issue again, I will upgrade to the latest version and confirm.

Thanks again!

  @Sunshine I am replying to this forum post, since the problem is similar to what I had, but on a different router.

I recently decided to purchase the Archer AX50 Wi-Fi 6 router, and so far I have been able to configure everything as my network required.

The only issue I am having, however, is that the WAN/network port is not communicating the correct bandwidth speed to the rest of my wired network.

I have tested the Wi-Fi 5GHz connection, and my devices are getting the expected 200Mbps from my internet service provider, but my wired devices is only reaching 100Mbps.

I have tried setting my Internet Port Negotiation Speed at 1000Mbps Full-Duplex as well as Auto, but this does not result in any change in the wired internet speed.

All devices on the network is either 1Gbps or 2.5Gbps capable.

I also upgraded to the latest firmware version for the device, and it is currently on version 1.0.14 Build 20240108 rel.42655.

Please assist.

  @Sunshine After leaving the router plugged in for about 18 hours, I retested everything and it managed the correct speeds. I'm unsure why several restarts did not resolve the issue.

Thank you as always for your assistance and willingness to help. I am a bit perplexed why information is obtainable in the Basic configuration mode, but not the Advanced version... I almost immediately switch over to advanced, as I normally customize settings quite extensively.