You can run the command ifconfig on the n900 to see what IPs you have configured. So using that you can see what IP is configured on the cell network interface (it is phonet0 if I recall). Mostly I think cell operators would give you an IP in a private address space, like 10.x.y.z or 192.168.x.y and send your outbound traffic through what's called a NAT gateway, so inbound traffic across the cell network would normally be impossible. I'm not sure, but it sounds from your post that you're trying to ssh to your public WLAN IP across the cell network? That wouldn't work because no-one (apart from you!) knows that IP is on the end of your cell network connection, so it can't be routed there. (Sorry if I misunderstood, it's a little hard to make out just what you're trying to do... Mentioning some example interfaces and IPs would help..)