@AntonCS I don't quite get your mind. But I guess u can use a dumb-switch to give out public IPs which from ISP. Like this:

ISP Modem --- Switch --- ER605

                          |

                       Other Routers or sth

Am I right?

  @AntonCS 

ER605 can't forward public IP to client devices. There are two solutions:

1. Connect your client devices to the main modem directly;

2. Do one-to-one NAT. The devices still get local IP but have full acces from Internet via specified public IP

How to configure One-to-One NAT on Safestream routers using the new GUI

  @HelsingL This would not work with the modem I am using. It has an option to use a cascaded router for distributing public IPs. But it forwards all traffic for that public subnet to the router, using router's internal IP on modem's private subnet.

  @Somnus Thank you, this answer makes sense to me. I've been at it for days and could not find a way to do this. Not happy with ER605 at all now :( But I guess it's on me. Should have bought a more expensive piece of hardware.