Not sure if you have read the previous posts on this type of issue or not, or checked out the [GUIDELINES] Submitting Bug Reports.I am not sure that the issue has to do with IP adresses. I am using the addon with 3 different devices, one using SPMC with KitKat, a tablet and a marshmallow based tv box with Krypton. The addon will not play on any of them. However, I can use a Web browser to log in and play on the devices on the same WIFI connection through the same ISP from the same IP address.
There are some sort of issues with the USTVnow service which can cause issues connecting to the streaming servers via their API. It can be hard to diagnose and it can seem like there is an issue with the addon since you can use a browser on the same network. However, I have been able to isolate the vast majority of these issues to some sort of defect in their system.
If you have a Free Account just create a new one.
If you have a Paid Account ask their support to move your account to a new server. Just don't mention Kodi or the addon since they will just say they don't support it.
I am assuming that your account worked previously in the USTVnow Plus addon. If so then the above steps should fix your issue. If your account never worked then it could be an issue with your login creds. For instance if you setup your account using Facebook/Google. That will not work with the addon.
I hope that helps.