I am having the same issue, and I believe I figure out the solution. Need few hours to verify! there seems to be a setting on the wireless connection “private address”, this settings keeps changing the MAC address of the phone causing a new DHCP reservation. In my specific situation my router logs show a long list of devices that registered themself as iPhones, each with its own MAC address. A new DHCP registration is shown about every 5-10 min until the router runs out of IPs, the issue is worse when you have multiple iPhones. who at apple thought this would be a good default setting to enable. I get the whole privacy setting, but perhaps a single MAC address per ssid would be more than enough.

Update: after disabling “Private address” option on the wifi connection settings, and waiting 24 hours for the DHCP leases to expire from my network wireless connection seems stable today.