WiFi access point without internet access – iOS devices disconnect

I'm trying to configure a WiFi access point to offer a web service on the local network, without offering clients internet access. The web server has a fixed IP address (let's say 192.168.2.2) and for user friendliness a local dns server assigns it the domain name app.com.

However, when an iOS 8 iPad connects to the network, it automatically tries to determine whether the WiFi network is "usable" – meaning it provides internet. If it fails, the iPad disconnects automatically.

We found this answer suggesting setting the DHCP server(udhcpd in our case)'s DHCP gateway to 0.0.0.0 ("null") and using static DHCP configuration. However, we wish to use DHCP, and indeed clients are no longer automatically assigned an IP when we unset udhcpd's DHCP gateway(router option). This is the relevant section of our udhcpd.conf, in case it helps:

start 192.168.2.100
end 192.168.2.149
max_leases 50
interface br0
remaining yes
option subnet 255.255.255.0
#option router 192.168.2.1

So, is there a way to tell iOS devices to stay connected to a WiFi network while still using DHCP for client IP's, subnet and DNS configuration?