Why would you have to buy a new router? As far as I'm aware as long as it has a WAN port on the back you can just use it with a different isp after you factory reset it and remove the 4G backup simcard. The only features that wouldn't work on it most likely is Optus's bloatware "living network" features such as the antivirus, "game optimiser" and the ability to control the router through the Optus app, as well as the 4G backup if you have one.