Em seu link está a resposta:
1st login After a successful install, assuming you and the device on which it was installed are on the same network, you should be able to reach the Nextcloud installation by visiting .local in your browser. If your hostname is localhost or localhost.localdomain, like on an Ubuntu Core device (IoT), nextcloud.local will be used instead.
Você deve abrir .local ou nextcloud.local no seu navegador.