Integrations not automatically discovered

Yesterday I did the first installation using the official Qnap container, which installs a very old version, the 0.65.5.

Today I removed that version because it was too old and I didn’t know how to update it, and I used an unofficial container as described in this article to install a newer version. Now I have the version 2021.2.3 installed.

The Onboarding page of the getting started says:

Home Assistant will show any devices that it has discovered on your network

After installing the old version (which had no Onboarding) I was able to see about 15 devices in my network. They were Google Home speakers, TVs, Rokus, ChromeCast TVs and I don’t remember what else.

Today, after installing the new version, I don’t have any devices.

Why is it not able to find any device on the local network?

I have changed the network settings on the container from this:
image

to this:
image

And now the local devices are visible.

I have no clue about the change I have done. I checked the network configuration, I changed one thing and it worked at the first try. I was just lucky.

Is the change I made safe?
Or am I risking to open the gates to anyone trying to get inside my home?

Docker requires host network for discovery

Discovery - Home Assistant (home-assistant.io)
specifically
“When running Home Assistant Core in a Docker container command line option --net=host or the compose file equivalent network_mode: host must be used to put it on the host’s network, otherwise mDNS and UPnP will not work.”