I'm trying to reach a state where my network is stable for weeks or months. I want silent, solid household network infrastructure that doesn't need my time and attention and coaxing to keep it running properly. I thought I was almost there. 

Not quite. 

For some reason, if my D-Link 890L router is up more than 9 days my daughter's Lenovo laptop (Windows 8.1 - she refuses to upgrade to Windows 10 due to the reduced privacy inherent in it) can no longer connect to the router via Wifi. It not only can't connect, it's actually forgotten it ever was connected...so there is no Wifi AP to "forget". It's gone. When i try to connect, it asks for the wifi password....and fails... 

......Just her laptop. 

The Macbook Air, HP Chromebook and ASUS laptops have no issues. Nor do the 6-7-odd Android phones / tablets.But for her laptop, my router could run just fine indefinitely.  

To rule out the device or Windows, her laptop CAN connect to any OTHER wifi AP, including various phones and the Huawei HG659b (in bridge mode - but 5GHz is left active for management).

The solution?  

All I need to do to get her reconnected is re-boot the router. Works every time.

But why? Why just her laptop?

I thought maybe it was the PC name so I changed it. Nope.

I thought it might be the MAC address, so I plugged in an ASUS USB wifi adaptor....and it can't connect either. Hmmm. 

I thought maybe it was the DHCP, so I reserved her IP address as static. At the same time I dialed the DHCP lease time way back from  7 days (10080 minutes) to 24 hours (1440).

She also keeps her laptop on hibernation mode rather than shutting it down.  I've wondered if this is corrupting her system's view of its DHCP lease time....but I don't know. 

Anyway.....after changing the lease time, the router required a reboot....and she was then able to get back on, as before. 

But what happens in 9 days? *Sigh*. 

I may buy her a new laptop.