The fire support module was not designed to be only usable within a certain range from a specific object.
The main design was that radio operators would have access to call in support where ever they are through their radio backpacks or hand-held radios.
What you can do to get around this issue is use a custom artillery class defined within your mission and use the unit_trait_required attribute within the class to require units to have the vn_artillery unit trait to access the radio support, the trait can then be added and removed during the mission how you see fit.
Here's the default class that is included within the CDLC, paste it into your mission's description.ext file and feel free to edit it how you wish.
https://pastebin.com/raw/3Fh47bQ9
A full documentation write up for the fire support module and the config class above will be available on the Savage wiki soon.
