[MANGA SPOILERS] RBA THEORY

Just a theory regarding how they infiltrated wall rose.

I think during the fall of shiganshina the female titan probably brought the mindless titans inside the walls and also helped remove bertolt from his titan after it broke the wall (she could have climbed the wall with her crystal claws, then jumped toward the nape and pulled him out) considering she wasn't shown at all she probably took bertolt towards reinners titan and RBA infiltrated wall rose by traveling underwater, through those water channels (the ones which the boats were flowing on) in titan form with beetroot in her mouth (lke she did with eren in the forest) and after reached end point close to wall rose, annie/reiner turned back into humans and went through the gates as normal children with the other citizens (eren/armin/mikasa) who got off them boats. (considering titans can move underwater/not drown)

Reason why I think under the water channels: After reinner broke the innerwall he stopped and stood near the water channels were E/A/M where traveling via steam boats. This is the only possible explanation of how they could have discretely got inside the walls without being spotted by the Manga Spoilers also the boats were already gone so they couldn't have gotten on them.