Just so, why does it say unable to connect to world on Minecraft Mobile?
Outdated network drivers may cause the “Unable to connect to world” issue. So you can try to update the network driver to fix the problem.
Additionally, why can't I connect to Minecraft servers? If you aren't able to connect to any Minecraft servers, it's likely a software issue on your end. Click Refresh on your server list (Java Edition only). If you are playing the Java Edition of Minecraft, click Refresh at the bottom of the list of servers. This often fixes minor connection issues.
Secondly, why can't my friend join my Minecraft world?
its possible you are using a different version of the game, insider program has a version different to the version you get through the store. make sure your game is up to date and make sure your friends settings allow friends to join.
Why can't I join a server in Minecraft PE?
Have you confirmed that your game and the host server you are trying to join are up to date and on the same version? Double check your app store for updates and ask the host of the server's if their games are up to date. This could also be an issue with your service provider blocking the Xbox Live port 3074.