Turning in a real head-scratcher, although that does point the problem to be more likely be your home LAN. Your D-link if not a little dated is still a more than capable model. If you haven't upgraded the firmware there's a 2013 update:
http://support.dlink.ca/ProductInfo.aspx?m=DIR-826L
Doubt that has anything to do with this issue though, but it's good to always keep your router up to date regardless.
Maybe try disabling IPv6. Even now as it's become more prevalent it can cause connectivity problems with Android.
That IP change shouldn't be an issue if you've forgotten/re-added the WiFi entry. Try booting one of those phones up into its Recovery Mode and wiping the system cache partition. I don't think WiFi setting info is even in the system cache but maybe there's just some corrupted WiFi-related data at a system level in the cache that needs to be cleared. And it won't really hurt to do this anyway, clearing the system cache is a non-destructive process that has nothing to do with your stored data.
Another thing to try is boot up a phone into its Safe Mode. While running in Safe Mode if it can connect properly to your WiFi network that would indicate some app you've installed that's creating a problem. Of course then you need to figure out just which one is the culprit.