If you already reach the Cerulean City after going through the Mt. Moon, there are no way of going back to the previous City. Simply because our character don’t know how to jump upwards, you’ll be stuck until you finish some further mission in the game.
However, You can still go back to these cities after you learn the fast travel in Pokemon Let’s Go. The required mission on unlocking the fast travel is in Celadon City. This is where you defeated the boss of the Team Rocket and by talking to a guy outside of the entrance.Â
Check our guide on this link, on how to unlock it.
Some people also suggested there are entrance in the Digglet Cave, but you’re required to defeat first the Lt. Surge.Â