Here is how I see it

They could possibly Reed mentions it at some point

It just subtle Easter Eggs

Or we do know the Fantastic Four will have to get to Earth-616 so maybe it will be shown or that will wait until Doomsday

Well the movie is set in another universe so it's a multiverse movie by default.

Yes. There were a couple of easter eggs on Reed's chalkboard.

Aye maybe 🤔

Reed talks to some kids about the Multiverse.

I want a uatu appearance in the beginning of the movie

Maybe the Fantastic Four will get teleported to Earth-616 near the end of First Steps

Most likely not in connection with Galactus. But I feel like they will bring up the Multiverse in the film.

I'm wonder if the earth of fantastic four will have a number in the beginning of the movie ?

@Antimonitor10 That is an interesting thought. Has there been any universe numbers confirmed in-universe since MoM? I feel like there probably have been that I'm not remembering.