ISPs do provide routers with firewalls, for this very reason of course. You still need to open ports to actually reach the service you're hosting, only now you don't need to figure out what your actual IP is.

There was a time the modem was commonly separate from the router.

Many ISPs do include a router in their modem with some wifi.

It’s often limited in some capacity in real worth use which lets customers over pay and undercover speed and be open to upgrading to a faster plan.