Your configuration is incomplete. First, your serial0/0 interface is "shutdown", so no traffic can pass. Should this be "IP unnumbered" or did the ISP give you an address for that interface?
Second, the "IP default gateway" command will not work. You need a default route.
If the ISP gave you a 206.217.239.x address for serial0/0, you will look something like this.
ip address 206.217.239.x (netmask)
ip route 0.0.0.0 0.0.0.0 18.104.22.168
I am just speculating on the configuration above based on experience. If you can indicate the IP addresses the ISP provided for each interface, I could be more specific. If you feel uncomfortable putting those in this forum, you will need to open a case.
Is this the configuration of your border router, or of the 2600? When you say you can "ping the default gateway", but cannot get out, can you get out to the internet on the default gateway? If so, it sounds like a routing or possibly an ARP issue. Or you can possibly clear arp on the default router.
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