It doesn't necessarily just work that way.
You need to have your main phone set up as the lead device first.
If that is done you might be able to change your ipad tariff to a sharer plan but you really should have done it when starting the contract.
I'd call CS now and see what they can do but unfortunately you might have to try a few times until you get someone who fully understands sharer plans
http://www.o2.co.uk/contactus
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