There is a simple way to adjust your server settings. For that you need to configure a local servero n your PC and get its configuration file.
Launche the game, click on host on the bottom right corner then parameters and create new parameters.
To make it easier name the set of config ServerPZ and adjust your settings. Once done you should find a ServerPZ_SandboxVars.lua file in your C:\Users\Your Windows username\Zomboid\Server folder by default (the exact path is given to you in game).
Now Stop your server from the RoxServers control panel then go to the file manager (or use FTP) and go to the folder server/Zomboid/Server/ of your game server. You will find a ServerPZ_SandboxVars.lua file. This is the setting file used by your server.
Upload the ServerPZ_SandboxVars.lua file from your PC to replace the one on server.
That's over you just have to start the server and the settings you made will be applied.