Force Android to use 3G when on local area wifi without net access

Question

I have a wifi LAN setup which does not have internet access. Just various other local wifi devices connected to it. The DHCP is configured to not return a gateway or dns ser

Answer 1

From code, when you detect there is no connectivity, you could switch off WiFi...

As for a setting, there is none (no good way to check if there really is connectivity universally and reliably). But some phones do just what you describe automatically, like for example my LG P-970.

(Note: Android disconnects from mobile networks when it connects to a WiFi, so there is no way to still be connected to a WiFi but route internet access through mobile, even though Linux can do it (with the ip route ... suite of tools))