I’m confused how you would be able to manage these devices if the WAN side of the router is unreachable.
Obviously this is the worst-case scenario but there may be cases where the WAN interface could be disabled and you would not be able to reach the device(s). In that case, you would either need to make the 5.5 hour drive (one way or two?) or have some means of out-of-band access to the router. In the enterprise WAN provider’s market, there is usually another connection other than the primary to provide access in the event that the primary link is down. This could be a phone line with a modem attached to the console port on the router. It would not necessarily have to be a dedicated line, but the customer could run a phone line to the device for remote access.
If the primary link is up and the customer permits access, you could use something like LogMeIn on their computer to troubleshoot the devices. But, this means that all devices along the path have to be operating and passing traffic.