I have concluded this because when the cable is connected to my laptop, Ookla reports approximately 60 mebibytes per second download speed, whereas via the router, approximately 500 kibibytes per second are achieved at best.

https://www.tp-link.com/uk/support/faq/3124/ doesn't assist me much, although it did inform me that the router appears to be correctly identifying the Ethernet connection, since the blue light of the relevant icon is active when the cable is connected.

https://community.tp-link.com/en/home/forum/topic/230166

https://community.tp-link.com/en/home/forum/topic/226372 (apparently inaccessible currently?)