Why did Zeke change from an Eldia enemy to an Eldia ally? Something I missed from the anime, or it wasn't explained, or it wasn't explained yet -- why did Zeke change from battling eldians to being their ally? I looked for an explanation in the wikia, but I couldn't find an answer there either. Is this explained in the manga? Why did Zeke change from an Eldia enemy to an Eldia ally?
Is this explained in the manga?

yes.

Why Zeke changed from an Eldia enemy to an Eldia ally?

SERIOUS SPOILERS WARNING!!
everything is going according to manga and you'll understand everything just in time.
I suggest you wait for the following episodes.
if you don't mind spoilers continue

Zeke had a goal, from the begging of becoming the beast titan. he wanted to change All of the Eldians genes so they can never bear any children. he wants to do that to stop all of the Hate and wars in the world (I guess :D). So, to do that he needs to contact the founder titan. so I don't think he was an enemy to Eldia he was acting so he could achieve his goal in the proper time.