You should have thought about this before you sold any device to any user. It's not guaranteed that you could keep the same IP address forever, even with the same ISP. Devices should have at least the option of updating the IP address remotely, and hopefully you can do this on a firmware update or alike.
The best option would have been, as already proposed here, to use a DNS name instead of an IP address. It would have allowed load balancing, redundancy and many other possibilities for extension. If it's possible through an automated firmware update, do so on your next update.
I don't know how many you have already sold but still: if it isn't, make it possible. We do know they already must have internet connection. Provide a cup of coffee during someone does the first firmware update manually, allowing future updates to be obtained remotely. Or don't and just wait while your competitors does this and takes your customers. Getting this right is part of your core business.