I have set up a point of sale station using a 3G iPad. I have a LAN printer that connects to a WiFI router that doesn't have an Internet connection. I want to use the 3G to connect to the Internet while being connected to the router's WiFi network for printing. It seems on the iPad the WiFi connection prioritises over the 3G dropping my Internet connection. Is there a way I can make this work for me?
No, this cannot be done.
The WiFi and the 3G connections can't be brought up at the same time on an iPad.
You could turn on WiFi when you need to print and get back to 3G after that, or use a service such as Google Cloud Print to print documents from the Internet.
The Google Cloud Print solution requires a computer connected to the Internet and to the printer.