Are you going over to their instance and trying to post through that? That’s not how this works. Take note of which instance you’re currently on though. The best metaphor I’ve seen is that this is kinda like email and you’ve got a Gmail account and you’re trying to email someone with a Hotmail account, you don’t go to Hotmail, you go through your Gmail account.

Assuming you’re on mobile, go back to the sh.itjust.works main page, press the menu icon on the right, select ‘communities’, under where it says “list of communities” select ‘all’, then search worldnews. There’s several options, select the one that’s on the right instance, and now you should be viewing the community while still on the shit just works instance. If you think you’d like to come back to that community in the future, there’s a subscribe option in the sidebar menu. This won’t work with all communities, but it should work with most of the larger ones (you’ll want to copy and paste something like !worldnews@lemmy.ml in the search bar instead, it’s a bit finicky on mobile though).