‘And the nations shall walk by means of its light (the light of the glory of the Lord), and the kings of the earth bring their glory into it, and its entrances will never be shut by day, for there will be no night there, and they will bring the honour and the glory of the nations into it.'
The city is made up of the people of God and among the people of God there will be kings and many nations (compare Isaiah 60:3: Psalms 72:11), and they will bring into the city all that they have which is best, even all their honour and glory. This cannot refer to earthly possessions for they have long ago suffered corruption and been left behind. This refers to their righteousnesses (Revelation 19:8), their honour, the gold, silver and jewels they received at the judgment seat of Christ (1 Corinthians 3:12), to the precious things they have stored up in Heaven through their righteous living (Matthew 6:19-20). These are they who are written in the Lamb's book of life (v. 27).
The point John is making is that not all kings of the earth are enemies of the people of God, not all of the nations are rejected. For some have wholly accepted Christ. There will be those of them who have their part in the holy city. (Revelation 21:27 makes clear that they are those who are written in the Lamb's book of life. This is no Millennial city).
The entrances will never be closed because entrances are only closed when night comes down and evil men begin to walk abroad, but there is no night here. The light of the glory of God and of Christ shines continually. All is light, transparent and open. The presence of God is continual and gives continual light (and there is clearly no need for sleep. The spirits of just men made perfect do not need sleep). And there are no evil men for they have no access as the next verse makes clear.