Have you tried with a different router? Perhaps this router has problems with the packets, high volume of uploads, or something else?
Have you tried a different WiFi channel? Does it have WiFi 5GHz to use instead?
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Is the iphone requesting an IP already on the network that is already being used by another device on the network.
Make sure it is set to DHCP (when connecting to wireless) and the router is giving out IP's in the correct range.
18,000 Address request a second, that's a few
Ok maybe set the iphone up to use a static IP address on your wifi
Try turning off IPV6 and see if that resolves it.