hornbyp wrote: Strange...

My daughter's IPhone 6S continues to function 'normally' - as does her old iPhone 5S. (The 2860 is now running 3.8.6)

I have a (useless) Netatmo thermostat in the house, which refuses to work properly with the 2860n, when set to certain channels (definitely channel #1, there may have been others). Swapping to channel #9 cured the problem. Worth a shot?

FWIW, here are my "advanced" Wifi settings, just in case they are pertinent :-

macximum wrote: I would set the channel bandwidth to 20/40 to enable better support for other/older wireless chipsets. (Please correct me if I am wrong).

Apple wrote:
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 neighbouring Wi-Fi networks. Routers that don't support 40 MHz channels in the 2.4 GHz band do support 20 MHz channels.

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.