To reply to a post in !GNUsocial you have to find a copy of it in your own instance. Older versions of !StatusNet pointed both the datetime URL and the conversations URL to the originating instance; I think newer !GNUsocial code has fixed that. As a workaround you can 'favorite' the notice, then it shows up under the "Popular" menu item https://status.obscuritus.ca/favorited/ Or you can do it the hard way and search through the DB for your local notice URL, type that into a browser, then reply from there.