Your ISP likely had something setup wrong and fixed it if this ever worked in the past.
This is why everyone who has a cable modem also has a router. Problem is people who don't have a clue use the terms interchangeably.
I am surprised you can get 2 devices to work most ISP limit you to a single IP that can be routed to the internet.
You need to buy a router or pay the ISP for more IP addresses.