If you have not configured anything and just connected the device with a LAN cable I suppose your device does have LAN IP.
I would recommend editing your end-device network settings to use static LAN IP. For example 192.168.1.100 , Netmask 255.255.255.0, Gateway: 192.168.1.1
When your operator changes SIM IP, it is inevitable that you will lose connection for a few minutes in the process.
Regards.