Assuming you are a UK customer and not based in the Republic of Ireland, where it looks like NowTV have updated their Help Article in the last day or so and there seems to be not that many JVC Fire TV models currently listed available with the NowTV native inbuilt App.
If the app is not available to download, then I suspect it hasn’t been supported yet from Now TV.
Unfortunately there is no confined date when the remainder of Fire devices will have the app available for download.
I have an JVC fire TV 49" and was disappointed the app is not yet available.
There appears to be no good way to contact the developer to find out when this will be released.
I cannot believe that if it's released on other 4k fire devices that the process to release on these TVs is overly complex.
I am working around it with a PC attached to the telly for now.
Some updated information would be appreciated!