Is Jio STB works with other network?
Well, I guess it depends on the apps you want to use. You might have to connect and register your STB with the Jio Fiber network when connecting it to the Internet for the first time.
Apps like Jio Cinema, Prime Video, Zee5, Hotstar and SonyLIV will probably work in other networks because these apps are not automatically signed in by the STB (using Jio Fiber) when you connect it to Jio Fiber. You'll have to login using mobile numbers or Jio Id (for Jio Cinema) to access our subscription for these apps. So, from the next time onwards, you can use other networks for Internet, and the aforementioned apps will most probably work.
However, apps like Jio Saavn, Jio TV+, etc,., which does not have a separate login (in the JIO TV version of these apps) might not work. Or they might work because you have already auto-logged-in once using Jio Fiber.
There is aalso the case of OTA Software Updates. You might not receive them if the STB is connected to another network. I am only guessing here. But it is what makes the most sense to me