Active Sync Time Outs

I have a small business server and i am getting Exchange errors relating to Windows Mobile direct push. According the MS Knowledge base it appears to be a http timeout.

“issue may occur if the firewall has not been configured to let HTTP(S) requests live longer than the minimum heartbeat interval that is configured on the server that is running Exchange Server 2003 SP2. By default, the minimum heartbeat interval at which the Exchange server triggers this event is nine minutes.”

The Mikrotik router is attached to a DSL and a NAT rule redirects the HTTPs request to the exchange server. What changes need to be made to the router to extend the HTTPS timeout?

Thank you