Process for providing cellular signal in underground bunker

I recently deployed a new site which is entirely underground in a bunker, it's relatively large (7 Cisco 1815i access points) and have recently been asked to find costings for providing regular cell signal (phone/sms) across the site.

I have absolutely 0 knowledge in this field, so wouldn't know where to start. It will be open to the public and is based in Holland (The Hague, specifically).

Any advice would be much appreciated.