If you need to show old messages always despite the fact that you enter a room, you should set the value on the other parameter named 'Max receive interval for public messages' instead of 'Interval for latest messages entering room'

This however will make it shows automatically old messages that are never been downloaded by a user. If you need to download old messages also if messages have already been download you need a little customization in the code.
Contact me by email in private for this.