Off Map Artillery

[Imp]

Not sure I understand I assume you are talking about the island getting off map artillery as the attacker will get naval.
The odd situation like this airdrops & mountain or jungle troops out of range are best covered by house rules or scenario design.
Mountain or jungle would most likely be restricted to mortars pack howitzers & maybe RCL, of course vehicles would also be restricted as in most likely none existent or pack horses if you want to class them as vehicles, hence house rules