The SIM card does not control the storage. The SIM card is only to identify the device to the network as yours, it is not a storage location. The phone has a designed capacity, if it started as an 8GB phone that's all you get and it can't be expanded. Yes, an SD card can be added but it seems few apps are designed to be able to be moved to the SD card, and even then it could only be a portion of the app that gets moved.
Sadly, 8GB is NOTHING these days and you may be in the situation of having to buy a new device as advised by the sales rep.
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