using my old iPhone as secondary phone w/o Internet access over mobile network

How do I switch “Internet access over mobile network” off in my old iPhone3?
I may want to hook on the Internet over Wi-Fi once in a while, but I must avoid “Internet access over mobile network“, as its current SIM card has no dedicated tariff for that any longer.

In my Android set-up I would go here:
Settings > Wireless and networks (More settings > Mobile networks > Mobile data (Enable data access over mobile network)

I have no idea, how to achieve the same with my iPhone3.

Any hints appreciated.