Then it's not just a modem. Do you have a separate router or just your "modem"?
If you have a router, then you're doing an unnecessary double-router setup. If you don't, then the whole point is moot. A modem is transparent to layer 3 and provides a common layer 2 among different layer 1s.
The (separate) router on the LAN side of the modem needs to be aware of the 192.168.100/24 on its WAN side or else it won't know how to reach it on layer 3, regardless of every traffic passing thourgh the modem on layer 2.