If it is a CDMA only phone, it is probably unlikely to work on the 4G network. They are now selling 4G LTE basic phones. It might be worth looking into.
I'm most definitely NOT a VZW employee. If a post answered your question, please mark it as the answer.
Phones that use CDMA and are already activated on a Verizon account will be supported until Dec 2022 for most people, although there have been reports of some cell towers, particularly in heavily populated areas, dropping service for 3G devices. In these cases, the cellular towers, which the communicators are relying on, are not directly owned or managed by AT&T or Verizon. These third-party providers are allowed to determine which technologies to support, and some are independently transitioning away from 3G before the Dec 2022 drop-dead date.