Issue - Network connection capped at 100Mb/s
Issue - Network connection capped at 100Mb/s
The issue lies in why Windows and Ubuntu automatically cap the download speed to 10/100/1000Mbps instead of the full rate. You mentioned clients achieving over 100Mbps on a connection that still relies on Gigabit Ethernet, which raises concerns about your motherboard's capabilities. Your CPU was released in 2017, meaning your board is likely several years old.
The router seems to recognize a 100Mb connection automatically. You may need to adjust the ports in the router's settings to switch between 10/100/1000 Mb speeds.
I purchased everything new in February 2019. The exact manufacturing date is unclear to me.