You can configure the gateway IP address, subnet mask and the default gateway in the Configurations / Backhaul menu item from the WMC dahsboard for every type of gateway (Wirnet iBTS, Wirnet iFemtoCell, Wirnet Station).
Unfortunatelly, you won't be able to configure the DNS servers from the dashboard.

To achieve this step, for Wirnet Station, you can change default DNS servers (212.30.96.123, 213.203.124.147) that are currently obsolete by using the following package custo_change_nameservers.tar.gz. Use it before proceeding to system upgrade. This package replaces default DNS servers by Google ones (8.8.8.8, 9.9.9.9).

For KerOS systems (Wirnet iBTS, Wirnet iFemtoCell), you should issue a remote SSH connection to setup DNS servers addresses before desactivating DHCP.

Refer to specific Wiki to setup static IP for your gateway:

Wirnet iBTS: http://wikikerlink.fr/wirnet-ibts/doku.php?id=wiki:backhaul_3.3.3
Wirnet iFemtoCell: http://wikikerlink.fr/wirnet-ifemtocell/doku.php?id=wirnet-ifemtocell:network_3.4.4