Sometimes, automatic settings don't work for various reasons. It should be possible to manually set an ip address.
For tracking purposes - Another user is also requesting this feature in the SUMO forums: https://support.mozilla.org/en-US/questions/1030655
We have a WebAPI to set static ip mode .  https://dxr.mozilla.org/mozilla-central/source/dom/webidl/MozWifiManager.webidl?from=MozWifiManager.webidl&case=true#222