Why can't I share 3G over Hotspot

I'm trying to share my 3G from my Samsung galaxy ace 2 to my iPhone 5 and it can't. I enable hotspot, it appears on my iPhone Wi-Fi list, I connect to it and it says it's connected but when I try to load something on my iPhone, it can't. It takes years loading and then says "Problem with connection" or "Connection lost" depending on the app I try to open.
My 3G is perfectly fine on the Samsung but doesn't load anything on my iPhone. It's not the iPhone because I can connect to my friend's Wi-Fi hotspot and it works perfectly, but mine just doesn't.

As i know for each iphone you need separate plan (contract) and pay for using internet separately. I don't think on laptop and iphone is the same internet.(except ipads)

Do you have a mobile data plan that specifically allows your phone to be used as a mobile hotspot? Does it work as a mobile hotspot for anything else (laptop, etc.)?

When I got my T-mobile 4G Samsung S3 I got the plan that slows down to 2G if I use more than 5 GB/month and can be used as a mobile hotspot, intead of the unlimited plan. So far I never used that much data because I have my phone connected to Wi-Fi at home or work.

I have used it as hotspot for my Win7 tablet PC, and have also used it for my desktop PC when I lost my DSL connection briefly a couple of times.

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