I think it looks likely that Windows IoT on the Pi suffers from the same problem that Raspbian did when the Pi 3 came out. WiFi channels 12 and 13 aren't legal everywhere, and someone didn't enable them in software.
There's a proposed fix on social.msdn.microsoft.com but there isn't enough followup on the post to figure out if it worked as intended. Note that 'MAX channel is 13...' is a typo which should read 'MAX channel is 11...':
Since there is no device manager UI available you can try changing the
channel directly through registry. From the INF file it says MAX
channel is 13, but its worth giving it a try.
1) The WiFi driver parameters reside under
On my machine N = 2. To verify it is the WiFi driver, the description
should be "Broadcom 802.11n Wireless SDIO Adapter". If it is not
\0002, look at the other entries.
2) Once you found the WiFi entry, change the "WFDChannelNumber" value
from "11"to "13". 3) Just to be on the safe side, change also
max from "11" to "13"
3) Reboot and check.
If it is the WiFi channel that's causing the problem, you can normally change it (in this case to something below 12) on your router's configuration page. You will almost certainly have to reconnect every other device in the house after doing this.