iOS and Macbook Pro/Air unable to join wifi network unless Network Isolation is enabled?

iOS and Macbook Pro/Air unable to join wifi network unless Network
Isolation is enabled?

I have a Netgear WNDR3700v3 wifi router, broadcasting two networks, one at
2.4GHz and another at 5GHz.
I have found that iPhone AND Macbook devices are unable to join the 2.4GHz
network (timeout error) unless the "Enable Wireless Isolation" option is
enabled - 5GHz network works fine (obviously not for iPhones though since
5GHz is unsupported). I'm not entirely sure what the option does apart
from prevent multicast/zeroconf on that interface.
I am using the following wifi settings for the 2.4Ghz network:
Region: EUROPE
Enable Wireless Isolation: ON
Enable SSID Broadcast: ON
Channel: AUTO
Mode: Up to 300Mbps
Security: WPA2-PSK [AES]
It's worth noting that I have disabled the built-in DHCP server on the
router, and instead use an Ubuntu-based DHCP server instead.
Any ideas as to why this might be a problem?