Hi Nicolas and welcome to Digi Forum!
Without seeing the actual configuration of the device, is a bit hard to help here. What I can see from your description is that you want to associate ETH1 port on both a LAN and a WAN interface and this is possibly the reason why is failing.
I would suggest to have a look at our userguide: https://www.digi.com/resources/documentation/digidocs/90002282/default.htm#transport/c_interfaces_ethernet.htm%3FTocPath%3DInterfaces%7CEthernet%2520interfaces%7C_____0
And also be sure to have latest firmware on the unit: https://www.digi.com/support/productdetail?pid=5617&type=firmware.
Then, if you still need help, the best would be to write an email to firstname.lastname@example.org and also check our support policies: https://www.digi.com/support.
Please be sure to include the IMEI/SN in your request.
Digi Technical support team