Thank you for posting your query in the Community. Securing your network with a device filter like MAC Address filtering is brilliant. We have discussed this topic previously in our Community if I correctly remember there is a limit of ten devices.
Is the modem freezing while you are trying to add the eleventh device?
The newest modems, used for the Ignite TV service (the Arris and Technicolor), have access to the ignite wifi app, which allows you full control over all the MACs for all the devices ,putting into profiles per users. Can lock all devices, rules, etc per user, then it locks all their devices.