I used the sidekick plan but I I use openvpn to connect to my home network. I also have squid proxy (internal) for net filtering and use that for n900, n810 and 2 laptops. It seems tmobile allows vpn on port 1194.
If you are just connecting from tmobile network, then you need open a port for squid on your firewall and use one of the open ports that the sidekick plan allows (25,110,143,465,587,993,995, 8080) and forward that to your squid proxy. Note 8080 is tmobile proxy so not sure if there will be a conflict if you use that port. I use port 995 for ssh/proxy. On the n900 connections settings, put this on the advance settings (ie. your_wan_ip port 995). Then you can surf the net, maps, etc.

I don't feel secure on the tmobile network so I vpn or ssh-tunnel to my home server to surf, usually. Also, make sure you have strong passwords on your ssh server or use public key authentication and move ssh port to other than port 22. Remember, the n900 is linux and can get rooted just like any other linux, usually through ssh.