1. Install a router between your ADSL Modem and a Windows machine
2. Use the Windows box (with IE or Firefox) to register the router with your ISP.
3. Connect your Amiga to the router.
If all else fails, I use this solution:
I made my amiga 500 (no ethernet card) a proxy client of my ISA Server (you can use XP and any free proxy app) and connect via null modem cable using MiamiDX.