@Elmar25 

No i dont believe you can do this as you cannot disable NAT on these routers to effectively "disconnect" any lans from any wan

What you could do, its double nat, get another router in between you ISP modem and your ER605 and have it set to a LAN with something like 253.253.253.1 which is highly unlikely your ISP will ever give you, you can then put that into ER605 WAN port and set a lan of 192.168.1.x as you need

This will double NAT though and there are some potential issues with that (UPNP wont work, will need to port forward any needed ports through both routers etc)

  @GRL I highly appreciate your reply. I thought that, as it is a professional router, it must be possible by realizing multiple virtual wans and lans. Thanks.

  @Elmar25 

It might be possible with Policy and Static routing, but i wouldnt hold you breath.  Remember, this is a pro-sumer / SOHO router thats $50, it cant do everything.

I dont think you can even easily do this with high end Draytek stuff and thats not even really "enterprise" grade gear