It is not exactly hidden or concealed the way Talokan or Attilan is, but instead, it is a physical country that has simply kept a low profile.

Source: https://screenrant.com/mcu-hidden-countries-locations/

On the mission where Okoye "lost" Shuri, the Princess was the one who requested to go to Talokan.

Source: https://www.cbr.com/queen-ramonda-okoye-banishment-wrong-black-panther-wakanda-forever-mcu/

The political and spiritual leader of Talokan, he mutated thanks to a Vibranium root his mother took while pregnant.

Source: https://www.cbr.com/aquatic-marvel-characters-could-featured-atlantis-added-mcu/