Which is the mainstream frequency band of 2G 3G 4G and 5G?

Frequency bands of 2G and 3G like 900 MHz and 2100 MHz can also use 4G if more subscribers move from either 2G or 3G to 4G. Recently, Indian telcos are awarded spectrums for 5G technology in both 3.5GHz (n78) and 700MHz (n28) bands.