iPhone 7 WiFi Connectivity Issue First apologies if this is not the best Stack Exchange site to ask this on, or this is "off topic", but I was directed here from Server Fault as it was "off-topic" for that site. I have asked this question over on Mac Rumors, but received zero responses at this time. Please feel free to redirect me to a more appropriate S.E. site if there is a more suitable one. 

I have seen many threads over the net about iPhone7 WiFi connectivity issues, but none of the answers for theirs have fixed my issue.

Problem:

We have an issue at home where the two iPhone 7s often will not connect or if they do connect, wont hold a connection for very long (10 - 20 minutes).

We have multiple other devices, both Apple and non-Apple who connect fine and hold their connection. It is only the two iPhone 7s which are experiencing this issue. The iPhone 6, iPhone 5, iPad 2, iPad mini, iMac, two Android phones, one Android tablet and Windows laptop all connect without issue.

WiFi Infrastructure:


Sky Broadband SR102, connected to internet and working as Fibre broadband modem, DHCP server and WiFi access point and router
Sky Broadband SR102, connected to SR102 by LAN cable, DHCP disabled, working as WiFi access point and router
NetGear WNR2200, connected to SR102 by LAN cable, DHCP disabled, working as WiFi access point and router. Also has Guest account enabled.


All WiFi access points have the same SID broadcast, the same encryption type and same password. All three broadcast on different channels, which only overlap slightly according to “WiFi Analyser” application. All WiFi is 2.4GHz. No 5Ghz in house.



Often when the iPhone has no connection the IP address is well outside of the DHCP range. Often something like 169.254.xxx.xxx. I understand from other threads this is an IP address the iPhone assigns its self when it cannot get one from the DCHP server. Sometimes the IP address is just plain 0.0.0.0.

Solutions which have not worked for us are:


Disconnecting and reconnecting WiFi on iPhone7
Renewing the lease on the iPhone7
Restarting the iPhone7
Forgetting WiFi network and re-adding it on iPhone7
Restarting all of the routers
Allowing SR102 to dynamically assign all IP addresses
Giving all frequently active devices in house static IP addresses in the device access list on the SR102. The static IP addresses are based upon MAC address. The  SR102 still hands out dynamic IP addresses to any other device.
Setting static IP addresses (to match those set on SR102) on iPhone7s
Resetting network settings on iPhone7
Turning Bluetooth off on iPhone7 (advice from forum posts - not practical and did not work)
Disconnect all IP6 DHCP on routers


We have also been to the Apple store and had twenty minutes with a genius, but after running a diagnostic tool on phone he was unable to offer anything other than what we had already tried except a full factory reset to new phone state, which is less than ideal for us.

I have seen posts around the net where other iPhone7 users having this issue can be sat next to partners with iPhone6, iPhone5, Android phones or iPads who can connect perfectly. It seems to be quite a common affliction to the iPhone7.  Please can anyone offer any other advice? Thanks in advance.
It seems as though it's an issue with your local network. You may want to take a look into your 2.4GHz settings, and change them to Apple's Specified 20 MHz bandwith.  


  2.4 GHz channel width
  
  Channel width controls how large of a ”pipe” is available to transfer data. However, larger channels are more subject to interference and more likely to interfere with other devices. A 40 MHz channel is sometimes called a wide channel, and a 20 MHz channel is a narrow channel.
  
  
    Set to: 20 MHz
  
  
  Use 20 MHz channels in the 2.4 GHz band. Using 40 MHz channels in the 2.4 GHz band can cause performance and reliability issues with your network, especially in the presence of other Wi-Fi networks and other 2.4 GHz devices. A 40 MHz channel might also cause interference and issues with other devices that use this band, such as Bluetooth devices, cordless phones, and neighboring Wi-Fi networks. Routers that don't support 40 MHz channels in the 2.4 GHz band do support 20 MHz channels.


If you choose to use the 5GHz channel width, it is reccomended that you use the 20/40 MHz band.


  5 GHz channel width
  
  Channel width controls how large of a "pipe" is available to transfer data. Larger channels are more susceptible to interference, and more likely to interfere with other devices. Interference is less of an issue in the 5 GHz band than in the 2.4 GHz band. A 40 MHz channel is sometimes called a wide channel, and a 20 MHz channel is a narrow channel.
  Set to:
  
  
    For 802.11n access points, set the 5GHz band to 20 MHz and 40 MHz.
    
    For 802.11ac access points, set the 5GHz band to 20 MHz, 40 MHz, and 80 MHz.
  
  
  For best performance and reliability, enable support for all channel widths. This allows devices to use the largest width they support, which results in optimal performance and compatibility. Not all client devices support 40 MHz channels, so don't enable 40 MHz-only mode. Devices that support only 20 MHz channels cant connect to a Wi-Fi router in 40 MHz-only mode. Similarly, don't enable 80 MHz-only mode, or only clients capable of 802.11ac will be able to connect. Routers that don't support 40 MHz or 80 MHz channels do support 20 MHz channels.




Source: https://support.apple.com/en-ca/HT202068