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That's not a phone issue, it's a network issue, 2G it's not supported in a lot countries anymore, you might want to contact them regarding that. As for 4G, are you referring to 4G LTE or the faux 4G (H+)?
If the former, is your SIM card 4G compatible? Do you have the correct APN settings? is your phone compatible with your carrier's 4G LTE bands ?
if the later, do you have the correct APN settings?