This auto configuration mechanism is called stateless address autoconfiguration (SLAAC) so "Your OS disable IPv6 SLAAC" or something like that in your favorite search engine should give you the answer. After that you should be able to manually configure your address and gateway like you would for v4. If your prefix is dynamic it gets a bit ugly then you have to use link local addresses and something like NPTv6 or NAT66 on the gateway. A better alternative IMHO would be to use different VLANs for the two networks but then your switch and possibly access point have to support it.