When the problem happens, and you can't access the Dlink UI, can you at-least ping it? Seems like it's not a connection issue between the Zyxel and Dlink, but the Dlink not responding. It could also be a IP conflict like I assume your thinking (because your changing the IP) but even then, what ever is conflicting should still be pingable.
Also, once it reconnects, is there anything in the logs explaining the problem? Status > View Log from the UI.
Last, when you say your PC is connected to the Zyxel, do you mean it stays online during the time your phone can't get online via the Dlink?