Yeah I fixed it. In the end it was pretty simple really (as always). I made the mistake of trying to use my Amiga's TCP stack from the emulation. That wasnt very smart. I dont think Genesis is designed to do this. Back in the days I used MiamiDX, too, since it did this job a lot better.
Well, I just didnt want to move to Miami so I tried something else. I just manually entered a new IP address in the same range as the Amiga is using: 192.168.137.9 (the Amiga has .2) this worked first time. Its just connecting to the pc (set as router) and the pc is handling the gateway stuff by means of network sharing on that adapter. So my router isnt even noticing the difference. Even better, since the Amiga is still online on another IP, I can use both Icab and Ibrowse at the same time.
BTW Icab is a pretty capable browser. Quite slow, but stable and the best browser I can currently use on 68K. (Netsurf is VERY unstable and slow, OWB needs blizkick removed and needs various older versions of ixemul and ixnet installed)