Using iPhone 4 to provide internet access to a router

We have an EftPos unit in the office which connects to our router to send transactions over the internet.

We are going to do an Expo soon, and I would like to use my iPhone to provide internet to a router, to which I can then plug the EftPos unit.

In other words, I want to use my iPhone to provide an internet connection to an EftPos unit, which doesn't have wifi capability and has to be plugged in by an ethernet cable...

Who is creative enough out there to help with a solution please?!

 

I'm not sure what a tethering plan is... I can use the iPhone as a modem for my MacBook Air by either plugging it in (with the sync cable) or connecting by wifi using the personal hotspot on the iPhone... does that mean I am heading in the right direction?