How do I enable mDNS on Hassio?

Hassio is not discoverable via http://homeassistant.local:8123, how do I enable this?

I believe that mDNS is supported by my router, as I can navigate to Tasmota devices by their mDNS name when I enable mDNS discovery on them.

Should be enabled by default on Home Assistant (Hass.io).

If you have a Linux system then use avahi-browse from avahi-tools to check if it shows up:

$ avahi-browse -alr

Using avahi-browse -alr I can see other devices resolving as .local, including my NAS and printer which I can access by using them, but hassio does not show as a .local