hi @JulK ,
When running external mode over ethernet, you need to make sure that the address specified in the IP address field is the same as the IP address specified in the Ethernet tab, in hardware settings:

The values from Ethernet tab are being used to setup the ENET interface. And the board will have that IP set up. And in the External mode, in IP address you need to specify the IP address of the board.
The values set in the Ethernet tab are just for reference - you can set up your network adapter with the desired address and modify there the IP and Gateway - the gateway must have the IP address you set on the host PC for the Eth adapter . - you'll need t Make sure that the ethernet cable is inserted in the right connector on the board - there is one for 100mbs and one for enet1g/qos.
I hope is more clear now.
Best regards,
Alexandra