"impossible to connect to the world" IMPORTANT

hi, i have a minecraft problem with multiplayer. I'm trying to play with a friend of mine, before we tried with two identical platforms, that is to say both of them from a smartphone and it doesn't work, now I have tried it with ps4 and he with a smartphone, I see it but when I load his world or the opposite he tells me "impossible to connect to the world" .. What can we do?