I can definitively say that the only way to access topics and feeds is through the web ui. Either the desktop interface or through the PWA on a mobile device would do it.
The reason no other apps have topic/feed support yet is because topics and feeds haven't yet been added to the piefed api. It's something that we want to do, but the api is still very new for piefed, and it is very much a work in progress, adding features as we get the chance to code them. I do know that it is something that the app developers want too (example). So, it will take some time, but I expect it to come sooner or later.