Using a firewall rule, we need to allow a certain type of TCP traffic from a PC on the LAN, to a cloud service on the WAN. The problem is the IP address changes frequently (it could be 1 of 50 different IP's).

Using a 2820vn we seem to only be able to specify a destination IP address (or a range) when creating a firewall rule / object, we cant specify a hostname as the target.

Are we missing something?

thanks

I don't think that's possible, you'll be best off just allowing those subnets/ranges using the specified port. The CSM is the only way to have the router distinguish hostnames but that's processed after the firewall, it can't be used by the firewall for filtering purposes.