When you say "pure router", I think what you're really saying is "I want this thing to function like a cheap wireless access point". An access point bridges wireless media to wired media (with many optional bells an whistles, too, on a real access point). I don't think you're looking for a second TCP/IP subnet and routing between it and your existing LAN subnet. I think you just want an access point.
My psychic powers suggest that you might still have a DHCP server enabled on the Encore device. Do you?
When you say "I can't access the net from the Encore router", are you saying "I am unable to access the Internet from a wireless client associated with the Encore router"? I'm not nitpicking, but rather just trying to be sure. I'm going to assume that you are saying that.
It sounds to me like the Encore is handing out DHCP leases to clients, and it won't allow you to assign "192.168.0.99" as the default-gateway that gets handed out to clients. I'm guessing you already have a DHCP server on your LAN, and you don't want two of those running on the same subnet simultaneously.
If this Encore is at all like the WRT54G's that I've used as el-cheapo access points you should be able to do the following and get it to act like a cheap access point:
- Plug one of the LAN ports on the Encore into your LAN
- Assign the Encore a static IP address for its LAN interface
- Set the WAN interface to DHCP client and forget about it
- Disable the Encore's DHCP server
- The Encore should bridge its wireless segment to the wired LAN, and clients of the Encore should get DHCP from wherever wired LAN clients get DHCP.
(I'm assuming you've got one subnet in your LAN and that the Encore doesn't need any static routes to other subnets, etc.)