I used Asus AM604-g as ADSL router. Not long ago I've moved from ADSL to LAN. Is it possible to use this device as ethernet router (I connect to LAN using VPN). If yes, how I need to configure it? Thanks!
I would be surprised if it is possible. The "internet" side of the ADSL router does some stuff that you need (DHCP client, NAT) and a bunch of stuff that you don't (ADSL translation, likely PPPoE authentication). You might be able to turn off PPPoE and make a cable to connect the RJ14 internet jack to the RJ11 on the Asus.
As I see it, you have several of options, listed in order of my preference. See this question for a discussion about home router/connectivity options.
1- Get a "standard" broadband router. They are inexpensive, easy to setup, and just work. I have had good luck with Linksys and Netgear.
2- Roll your own router on a spare PC with two NICs. I would use Astaro, although other options are noted on the question I linked above. You can use the Asus as a switch and wireless AP behind it.
3- Plug a PC with wired and wireless into the ethernet internet jack and get it working. Then set it up to share the internet connection, and use the Asus switch capabilities to connect the other computers to the gateway. THIS IS A KLUDGE .. I would not do it permanently.