You need to pass the Context (from getContext() or "this" if it is an activity, as a new parameter to the static method and call context.getSystemService.
public static boolean isNetworkAvailable (Context context) {
ConnectivityManager manager = (ConnectivityManager) context.getSystemService(Context.CONNECTIVITY_SERVICE);
Basically, you are probably used to calling getSystemService from a class that extends Context like an Activity.
Now from an activity call something like if (appHelpers.isNetworkAvailable (this)) {
or
if (appHelpers.isNetworkAvailable (getContext ())) {
depending on where you are calling it from.
You can make the helper methods static, or add a constructor to your helper to pass in the context and have the helper keep the context as a member.