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> Now I am back and I change sim card back to public mobile s
What specific "public mobile" are you referring to? Do you mean your home country's SIM card or something else?
> but my phone is not working saying mobile net work is not available
If you are referring no mobile network or no data access after switching SIM card, please try resetting your network settings in iOS settings (if you are using an iPhone) or manually resetting mobile network APN in Android settings (if you are using a Android phone, please advise your mobile company so we can help you find the right settings accordingly).