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If you have 2G only plan, try going to Settings -> More -> Mobile networks -> Preferred network type -> GSM only.
WCDMA is 3G, if you are on 2G plan then you can't access that. Either set your option to 2G only or contact your service provider to upgrade your plan.
I dunno how this happened, but you said you have a 2G plan. How do you expect to get 3G? It's all depending on what's the provider allowing.