Aw, that's too bad. are you okay with digital? because I can only direct you to Yen Press (the official publisher) - once you're caught up, the newest chapter is free until the next one releases.

The Summer Hikaru Died (manga serial) | Manga | Yen Pressyenpress.com

If you have discord, join our affiliated server and say you're having trouble getting the volumes or you aren't sure of the most affordable way to read it. someone will help you out for sure.