A 5G SIM can be used in a 4G handset but will work in the 4G network only. In India, all the telecom operators have rolled out the 4G network and one can use that 4G SIM in her/his 3G handset. The difference will be the speed in data communications as the communication protocols of the higher generation (4G, 5G) networks are not supported by previous generation handsets.
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