We live in an heavily populated neighborhood with hundreds of wireless routers within a one block area. This means that almost everyone has their wifi router base channel set to 11 - delivered that way in the box.
Can you see the signal for your wifi router, yet your devices - iPhones, laptops - cannot communicate and connect to your wifi network? Then connect your PC with a cable to your router. Open your browser and go to your Router Configurator http://192.168.0.1 (check router label for this exact URL).
Go to your Wireless Basic Settings and change the channel.
Your devices are just having trouble connecting to your wifi signal with all those routers signaling on the same channel in such close proximity to one another.
Now, unplug router and modem. Restart your modem first; wait a few minutes and restart your router.
Hope this helps. Many thanks to Rick at UPC NL call center for working through this with me. What a relief and such a simple solution.