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More about LTE Coverage and SIM
Once you move outside the LTE signal coverage, your LTE device will connect to the next highest technology it can find like HSPA+ or 3G.
No, you will still be using the same number.
Yes, you can use your LTE SIM in a non-LTE device, but please note that a non-LTE device cannot provide you with LTE speed. Instead, it will connect to the next highest technology it can find.
Yes, an LTE SIM will work while roaming.