How to set static IP without specifying gateway

I'm trying to set up a Wifi with a static IP but NO gateway or DNS but until I enter a gateway and DNS the save button remains greyed out.
From a networking perspective, you should be able to have a Wifi connection without a gateway (This is true for all devices with multiple network connections) so that it uses an alternative connection (Mobile data in this case) but for some reason the Android devs seem to think otherwise.
The network I'm connecting to has no internet connection (Used for local connectivity only) so I need the phone to still use my mobile connection whilst connected. As Android turns off mobile data when connected to Wifi I was hoping I could leave the gateway blank to force it to use it's mobile connection when connected to this network but I can't
How can I either have mobile data on for just 1 SSID or save a connection with no gateway so it does use mobile (I tried to no avail)?
Phone is a rooted HTC One M8 running Android 4.4.4

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