The Upaniṣads were preserved through an oral tradition and are therefore recited. Yet unlike the core Vedic verses (Saṁhitās) or specifically prescribed mantras, this genre of scripture is not meant for ritual chanting.
There are exceptions, but as a rule, the Upaniṣads are not chants—they are spiritual teachings.
From a modern perspective, you might think of each Upaniṣad as a “publication” by a particular sage or lineage. These are their insights into the meaning of the Vedas, which they recorded and handed down for seekers.
You wouldn’t pick up any book in a bookstore and begin chanting it, would you? In the same way, the Upaniṣad genre is to be approached through study, reflection, and contemplation rather than ritualistic recitation.
At their core, the Upaniṣads are for śravaṇa (grasping the teaching), manana (reflection), and nididhyāsana (deep assimilation). That alone is their proper and faithful treatment.