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Every temple in India has at least one priest. The act of preaching scriptures a recommended act or mandatory act for him?

In the Smritis, the temple priests are in fact considered fallen/degraded. Probably because they earn through the act of worshipping.

Manu Smriti 3.152. Physicians, temple-priests, sellers of meat, and
those who subsist by shop-keeping must be avoided at sacrifices
offered to the gods and to the manes.

3.180 (Food) given to a seller of Soma becomes ordure, (that given) to a physician pus and blood, but (that presented) to a temple-priest is
lost, and (that given) to a usurer finds no place (in the world of the
gods).

Alternate translation from Wisdomlib.org:

सोमविक्रयिणे विष्ठा भिषजे पूयशोणितम् ।
नष्टं देवलके
दत्तमप्रतिष्ठं तु वार्धुषौ ॥ १८० ॥

somavikrayiṇe viṣṭhā bhiṣaje pūyaśoṇitam |
naṣṭaṃ devalake
dattamapratiṣṭhaṃ tu vārdhuṣau || 180 ||

What is given to the Soma-seller becomes ordure; to the physician, pus
and blood; that to the temple-attendant becomes lost; and that to the
usurer has no place.—(180)

The link also gives a similar verse from Mahabharata which is as follows:

Mahābhārata (13.90, 13, 14).—‘What is given to the Soma-seller and to
the physician becomes pus and blood; what is given to the
temple-servant perishes; it is neither here nor there.’

So, it is certainly not their responsibility to teach scriptures.