Same problem here. I’m activating the hotspot on the iPhone and connecting to it through the admin panel in the Repeater section. I have also tried to use the openwrt luci thing to connect to it.
The connection takes quite long to establish (order of 20-30 seconds) and then drops within 5-10 minutes and the hotspot is disabled. On the iPhone, the hotspot is disabled automatically after some time without any new connections.
On the other hand, connecting a Windows laptop to the iPhone hotspot and then activating the hotspot on the laptop and only then connecting the AR750 to the laptop, effectively routing iPhone internet through the laptop first before connecting it to the AR750, results in a stable connection that does not drop.
TLDR:
iPhone → AR750 = unstable
iPhone → laptop → AR750 = stable
So perhaps there is some TTL or some other setting in the openwrt which can be modified to keep the connection with the iPhone.