Arris C3 modems not accesing the internet

We recently bought a used arris c3, it has the ospf license installed. We placed a new configuration in the cmts, and all the cable modems get provisioned but they don't access the internet.

We make ping within the cmts cli to any internet ip and get response, we can also ping any cpe from the cmts. But from the cpe we can only ping the cmts not any internet ip.

Unfortunately we don't have any arris support contract.

If someone could please help us!!

Thanks,

Adrian