Thats how they function. Because its a modem only device and not a modem/router combo device.
most isp's only allow 1 public ip address. (cox/xfinity/spectrum/+ others)
Certain situations allow more IP addresses assigned like if you're on a business class line, if you pay for extra IP addresses, or if you're isp is one of the lucky few who allow more than 1 public ip address.
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