ive never used the trend fw, but it should have the ability to add a "trusted zone" or "trusted network" to which you could apply your activesync connection.
see this link for more information
ActiveSync 4.x Troubleshooting Guide - Firewall, VPN and Proxy
and eventually this link
ActiveSync Help & How Tos
and you should use comodo or outpost firewall... :)
jd