Of course if port 91 is not enable on the packet filter rule it won't work. Of you use Http proxy , you will probably have to add it to be filtering advanced tab as allowed target services.
Is this website in question on the inside or outside of your UTM? If it's inside the UTM (on your LAN) and you want outside users to access it (you host the webpage) then you'd need to use a DNAT.
If the however the site is on the outside (the Internet), which I going to assume is the most likely case, you need to setup a firewall rule allowing traffic destined for port 91 to pass. I'm not near a UTM right now so not sure where the firewall rules are sitting but for any outbound rule, you'd want to setup the following rule:
Source: Internal LAN (Network) Destination: Internet Service: YourService
YourService would be the name of the service sitting on Port 91. If it's not already created, you'd have to create a service definition. When creating the definition, set the destination port to your port 91 and leave the source port at it's default (1-65535).
