How to set up an Internet connection through a modem that is connected to the RPI, but itself is connected to the router?

Question

Computer networks

Hello! I have a question: How to set up an Internet modem and RPI So that both distribute the Internet to the router (as in the picture)

Answer 1

configure port forwarding on RPI, set proxies, IP tables, a lot of options. What OS is on Rasbery?
And in general, why is the modem in the RPI, why not plug the RPI into the router, and the modem there, and not set everything up according to the normal scheme?

Answer 2

Put on RPI openwrt for example. and you can use it as a router. and everything will be much easier.