>>/956255/
> why the router doesnt have the public IP address and the modem does
Because you have requested that ISP assign a static IP address to the interface of the modem. If you want the router with a public IP address, you will need a modem router combination like this:
https://techspecs.ui.com/unifi/cloud-gateways/ucg-max

The ISP is only responsible for the connection into the modem, how you connect it onward is not their responsibility. If you get something similar to the above device, then you are responsible for the entire configuration. The ISP doesn't want to provide Level 2/3 Technical Support for you because it would cost them much more money.