"Looks like the cause of this is commit fb473a7 ( https://github.com/EpicGames/UnrealEngine/commit/fb473a7678c3669bebe0431dacaf52bcf8dee47f#diff-e6f727710008f31c0ece339b76041d2e ) included in v4.23.
Android and Windows clients connect to Linux and Windows dedicated servers without any problems. iOS clients refused to open connection to both Linux and Windows servers. We tried mobile internet with server in the cloud and local wi-fi network - no luck in both cases. No log messages on the server side, no application crashes on the client side. When we build our project with older version of the engine (4.22 and earlier) iOS connections work as expected."
Head over to the existingAnswerHub thread and let us know what's up.