Learn Hardcore Italian: Durante la manutenzione l’ascensore resterà fuori servizio per due ore. - During the maintenance the elevator will be out of service for two hours.

durante is a preposition meaning “during” or “throughout,” and it must be followed by a noun (here la manutenzione).
By contrast, mentre is a conjunction meaning “while” and is followed by a verb clause. For example:

“Mentre l’ascensore è in manutenzione, prendete le scale.”
You cannot say mentre la manutenzione because mentre cannot introduce a bare noun.

Why is restare used instead of essere?

restare literally means “to remain” or “to stay” and focuses on the continuation of a state.
You could substitute essere without changing the basic meaning:

“L’ascensore sarà fuori servizio per due ore.”
But notices often use restare to signal “will remain out of service,” stressing its ongoing condition.

What tense is resterà, and how is it formed?

resterà is the third-person singular of the futuro semplice of restare.
Formation: take the infinitive stem rest- and add the future ending -erà.
Full conjugation (singular):

io resterò
tu resterai
lui/lei resterà

What does fuori servizio mean, and why is it structured that way?

fuori servizio is an idiomatic adjective phrase meaning “out of service” or “out of order.”

fuori (“outside”) functions like an adverb here, modifying servizio (“service”).
Together they form a fixed expression used for machines, elevators, etc., that aren’t operational.

Why is it l’ascensore instead of lo ascensore or la ascensore?

Ascensore is a masculine noun, so it never takes la.
The masculine singular definite article il elides before a vowel, becoming l’.
Lo is reserved for masculine nouns beginning with s+consonant, z, ps, pn, etc., not simple vowels.

Why is there no comma after manutenzione?

In Italian, short introductory prepositional phrases often do not require a comma. Commas are optional here and depend on rhythm or emphasis. In a brief notice like this, omitting the comma is completely normal.

Why per due ore instead of per due ora or tra due ore?

ore is plural because the duration is two hours.
per indicates the length of time (“for two hours”).
tra due ore (or fra due ore) would mean “in two hours” (i.e. how long before it starts again), not the duration.

Why is manutenzione feminine? Is there a rule?

Yes: Italian nouns ending in -zione (from Latin -tionem) are always feminine. Therefore the correct article is la: la manutenzione.