cant join my friends minecraft world

Dear Mojang Studios, Im really upset because for more than five days in a row i can't join my friends world where we were playing togheter. I've been trying to re-sign into my account but it doesn't help. Every time i try and join it says that it's "unable to connect to the world". Im sure we're playing on the same version "v.16.101" and on the same tipe of iPhone "iOS 11". Would mean a lot if your team could help us. Thank you.