I am using the Adafruit Ethernet FeatherWing board using your W5500 chip. The board is connected to my network via a NetGear Gigabit Hub. I am having issues with the auto-negotiation not working correctly all the time. It sometimes negotiates at 100Mb but most of the time it negotiates at 10Mb. I am using the LED’s on the Hub for the speed negotiated with the hub.
Look into the logs of the hub if it has these. Change cable. Change hub. Change Ethernet board. If problem remains then configuration of W5500 or hub/switch is incorrect. If problem will disappear then most probably last changed device is the problem.
Did you connect the RST pin of your FeatgerWing board to your MCU?
Check the datasheet of W5500 how to properly reset W5500 after power on.
In case you did proper reset then this could help: W5500 No PHY Link