Probably the magazine has to be federated first - and so it has to be searched from the instance. I'll provide example using magazine AskKbin and assuming you're using browser version of Lemmy. The local adress for magazine is https://kbin.social/m/AskKbin
Go to your Lemmy and press search button. Search for the magazine using the local adress of the magazine (https://kbin.social/m/AskKbin). Once searched, it should start federating and you can subscribe to it. You should be able to easily subscribe to the magazine as well - as it is going to appear as community in search results. Similiarly you would initialte federation with communities from other instances of lemmy, if they aren't federated yet

@PeanutsBeside Some magazines (KBin equivalents to subs on reddit, called "communities" on Lemmy) are synced through activity pub in this federated system. So the resource you browse is not actually on KBin, but it might be somewhere else - for example some lemmy instance like lemmy world. Those resources are not synced since creation, but rather from the moment it was first looked up here. Also not everything is synced if nobody is subscribed to said federated community. If you would like to see the content on original instance, you can open the thread here on kbin, and under the post there are options - look for one called Copy Url To Fediverse, then open the link in new tab in your browser - you'll see where the post originate from. It's just a nature of federated communities. I'm not sure if all the old posts will be synced - likely not - but you should be able to see new posts