Ok so this is basically how it works:

You can search by keyword in the instance community list, but that's only going to turn up communities that the instance you're searching from already knows about. With .world being so big, we've already done most of the hard work telling it about places, and now it knows about most of them so search results turn up a lot of stuff. On a smaller instance, it's more likely that you'll be the first to try and access a particular community elsewhere.

So first, congrats, you're a pioneer for your new instance!

What you need to do is search with the !community@instance syntax on the new instance, rather than just keyword search. Note the ! at the beginning of the search term. So for example to find this community we're in right now, you'd search for [!general@lemmy.world](/c/general@lemmy.world)

You might need to give the search results a few seconds to update, normally it'll say no results but then auto-update as it forces an index of the new community. The cool thing about this is once one person from an instance goes through this process, everyone in future can simply search by keyword and it'll show up as expected. So you're really doing your new instance a solid.

In your situation you're just trying to duplicate your sub list so you already have all of the addresses, but I can also recommend lemmyverse.net/communities for finding ones all over the place (including probably plenty that even lemmy.world doesn't know about yet).