Razer Diamondback Plasma buttons

Hello! It's my first time posting on these type of forums, but to the point; I have a Razer Diamondback Plasma, that I got from my boyfriend. It's a very old model, it's not being supported through Razer Synapse. It's working dandy, no problems but one. I can't assign the buttons on the sides to work as the "previous page" and the "next page" buttons. Buttons do work (I was able to use their in games), but somehow I can't assigne them in any ways for the browser. Does anyone have any tips? I have the RazerDiamondback app, but it doesn't do anything for the buttons.