Namor isn't Wakandan. For their interpretation of Atlantis, the MCU is making them Aztec/Mayan themed. The dude playing him is Mexican.
They're not doing a battle for the throne thing, with Namor at least. Wakanda battling Atlantis is pretty common in the comics. From the trailer, it seems Wakanda with their advanced technology accidentally discovered Atlantis and now shits going down.
I agree that the actor is Spanish and they're using a Mayan theme, but based on the trailer I got the impression he's half-wakandan. Is it confirmed yet what his origin is somewhere?