Hummm .... That is strange.
I.ve never had it happen (yet) but I follow a set procedure when making changes.
A couple of points:
Hiding the SSID really isn't effective. The same with whitelisting IP's, they can be spoofed. While it does add one more "layer" it is only effective in keeping honest people out.
Whenever I institute a change I use the following procedure:
I connect a single computer by wire to the router.
If changing the routers GUI username/password, I do that from the wired computer and then reboot the router. Doing so will give the router a "clean slate".
There is one place things can get tricky ... having a browser such as Firefox remember usernames and passwords for anything router related ... don't do it.
If Changing anything wireless related I will take each wireless device in turn and delete the connection. When all wireless devices have been "uninvited" I will change the wireless encryption, save the changes (doing this from a wired connection) and reboot the router.
That mandates that all wireless clients then start from scratch beginning with SSID discovery.
Again I've never had your issue.
I'll be interesting in seeing what others may post on this issue.
Sorry, but I have absolutely no clue. I can see taking a minute for the router to reboot, but not to have no access for days, then all of a sudden it just works. That's REALLY odd.
It's Gremllins, I tell ya, GREMLINS!!!!