Changing firmware from ISP to Zyxel

Dear community, I hired the services of a ISP here in Switzerland. Then I stopped their services and they told me I can keep the router. Its a AX7501-B0. The problem is that it has a custom restricted ISP firmware. How can I change the firmware and how do I do this? 
Thanks anyone for pointing me into the right direction to be able to use this device.

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