Changing the network configuration through the WMC dashboard is not recommended since DNS configuration cannot be achieved.
Refer to these specific Wikis to setup static IP for your gateway:
Wirnet Product line (iBTS, iFemtoCell): http://wikikerlink.fr/wirnet-productline/doku.php?id=wiki:network_mana:backhaul#ethernet_configuration
Wirnet iBTS (before FW v4.0) http://wikikerlink.fr/wirnet-ibts/doku.php?id=wiki:backhaul_3.3.3#ethernet_configuration
Wirnet iFemtoCell (before FW v4.0): http://wikikerlink.fr/wirnet-ifemtocell/doku.php?id=wirnet-ifemtocell:network_3.4.4#ethernet_configuration
Wirnet Station: http://wikikerlink.fr/lora-station/doku.php?id=wiki:faq#how_to_configure_static_ip_address