The error usually indicate a network problem.
To rule out the DNS problem, try to ping from another location to the public host name of the ISA server. If the address it trying to ping is what you expected, then you DNS is working properly. It is typical for DNS update can take 4 hrs or more. If it doesn't work initially, wait 4 hrs and try again. There are other techniques to find this out sooner, but it is more complicated.
Once you determine it is not DNS problem, then you need to look at network issue. Does your ISA server have two network cards? You indicate "external card", so I assume you have more than one card. What is the IP address of this external card? To protect your privacy, you only need to let me know the first two number of the IP address, e.g. something like 192.168.x.x.
Since you are able to telnet to your 877 router, please post the output of:
show run | i nat
Again, to protect your privacy, only show the first two number of each IP address.