Not familiar with Mediacom. Is the Arris their's (you rent) or yours (you bought)?
Could be that that feature is not available via the firmware. Not unusual to have some features and functions disabled on ISP provided devices.
In fact, I sometimes think that all ISPs, modem manufacturer's, router manufacturer's etc. would be more than happy to disable everything from the end user just so the ISP or manufacturer could charge for configuration services. (Cynicism conceded.)
Not sure what else to suggest. Hopefully some other reader will be able to spot something I missed or provide a viable option. No problem with that on my end.