washere, I am able to reproduce your issue with my GL-AR300M16. After reset, I am able to install Advanced → LuCI and File Sharing → File Sharing or Multimedia File Sharing (DLNA) but not both.
In which Network Mode are you? I am able to change the SSID via the GL Web user interface → Wireless. By the way, you can change the SSID not only via the Web interface but also via SSH and there via UCI ….