it's very hard to guess the issue, it does seem configuration-related.
After the router was changed out, was something done with the device from the customer's location? Perhaps it has not been reset and still has configuration on it?
Let's hope that it does not happen again, however, if it does, please navigate to System → Administration → Troubleshoot and generate a Troubleshoot file. This will give us some more insight into what the issue could be.
And just to clarify, there was only one public IP that was unreachable from the RUT950? Internet browsing worked just fine?