This entirely depends on your various Character's motivations
If the tribe is not that devout or maybe only a small number of them are then some simply explanations and demonstrations of the principles of the pre-apocalpyse tech that gave rise to the stories that in turn gave rise to them being Gods, might be enough.
or
he'd have to convince the tribe that entering the city will not anger them and therefore they have nothing to fear and demonstrate that
If he refuses to lie to the tribe, but happy to bend the truth
then he will simply have to tell the tribe he will not enter the city of the gods leave the other way and then go round them. i'm presuming the tribe have not built a huge circular city of their own around the ruins??? being that the city is not actually a city of the gods, then his statement is true, while also deceiving, hence bending the truth
If the MC is happy lying
Then as above saying he has divine permission is the simplest way to progress
If the MC refuses to lie, and refuses to bend the truth and go round them, and the tribe devoutly believe it is the city of the gods...
then he'll need to setup and schooling system, educate the young of the tribe so they can grow up to educate their offspring, develop suitable water treatment and farming to improve upon the tribes lives, then force the tribe into an industrial revolution, where they can begin to build basic versions of the pre-apocalypse technologies, throw in a couple of world wars, to further enforce the development and even more advanced tech, learn to split the atom...
Then have a neighbouring tribe start a complicated game of oneupmanship, until is finally ends with the tribe landing on the moon...
then after another Generation or two, they may have a few tribesmen, usually teenage boys trying to show how fearless they are to impress a girl. might venture into the city. once this has been done and the MC (which would need to be very very long lived) might be able to convince those same boys that would have since grown up that its was safe and the old very devout members of the tribe have all died of old age, then maybe they could be convinced with reasoning... but scientists have been trying that for centuries.
For the majority of mankind, their beliefs will always take priority over fact. to use an above example:
Evolution is provable fact, and yet creationists still exist...
The earth being round is provable fact, and yet flat earth believer's still exist...
DCEU movies have pretty much all been utterly terrible is an obviously provable fact, and yet supporters still exist...
These are not criticisms of the believers, just simple facts...