The date that you can expect to lose your 3G or 4G access depends on your carrier. Each has listed the following sunset dates for their services:
AT&T: The AT&T 3G sunset date is February 22, 2022.
Verizon: If you're a Verizon user, the 4G sunset date is December 31, 2022. However, the carrier will shut down its 3G service by the end of 2020.
T-Mobile: While T-Mobile hasn't confirmed its 3G sunset date, most industry experts expect it to occur at the end of 2021.
Sprint: T-Mobile and Sprint have officially merged, but they will continue to operate as separate entities. Sprint will terminate its code-division multiple access (CDMA) 3G networks on January 1, 2022.