After 3 days of struggling, when I finally decided to look for help and write this post, I end up finding the solution the same day.

If anyone is in this situation, try to execute the following command:

$ sudo nova-manage cell_v2 map_cell_and_hosts

Ref: https://docs.openstack.org/nova/latest/cli/nova-manage.html#cell-v2-map-cell-and-hosts