The WL-USBWIFIRPT-3000 is basically your standard home router, but it can be powered from 12V with a cigarette lighter adapter accessory (sold separately).
The antenna on it is where your boaty devices (laptop, tablet, phone, etc.) connect to. To get internet into this box, you can hook up a USB 3G dongle or USB wifi adapter, which in turn may have a big honking antenna roosting on your spreaders, or just a little one, that's up to you. There's an appendix at the end of the manual[1] listing compatible USB 3G dongles, and for USB wifi adapters the Solwise page says it works with adapters that have a Ralink RT3070 Chipset or possibly a RT3050 or 8187L. That includes the Rocket antenna (which has a built in USB wifi adapter).
Once that's hooked up, it can do standard home routery things like NAT, making all your devices look (more or less) like just one to the internet provider.
[1]
http://www.solwise.co.uk/downloads/files/wl-usbwifirpt-3000-user-manual.pdf