I had a similary problem when i configured my RPI as a AC.
I think, the solution is as:
Try to stop or disable the preinstalled dhcpcd client because not, you cannot configure some interfaces in /etc/network/interfaces.
configure the wlanx in interfaces as ip-address and broadcast
auto wlanx # that means wlanx start on boot put the number what you have instead x. Usually is wlan0
iface wlanx inet static
ip-adrress=10.10.10.1/24 #as example. that 24 is for netmask as bellow defined
netmask-=255.255.255.0 #that means the network class is between 10.10.10.1 to 10.10.10.255
#you dont need to configure gateway
#you must configure the eth0 interface here as static ip because you have not dhcpcd for autoconnect to another server or somewhere to #internet. I think, except is the connection is ppoe
#at last you must modify in dhcpd.conf the INTERFACES="wlanx", not eth0, because wlanx offer connections to other devives as your phone. If not, the phone cannot get an ip-address from your RPI as server.
I hope that help !