The second one:
En dessous de l’escalier.
Note that you can also use en dessous on its own, while sous should always be followed by something. For instance:
Le livre est-il sur la table ? Non, en dessous. Le livre est sous la table.
As for the sentences you propose with just dessous (not en dessous), they are both correct (although le chien est sous la table sound more natural to me).
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