How can I allow a VirtualBox machine to reach the internet but bot LAN devices?

I need to setup a VirtualBox machine that:

1. can connect to the internet;
2. can't connect to the devices in my LAN.

If I understand the manual, I should reate a NAT Network from File > Preferences > Network and then set Settings > Network > Adapter 1 to:

• [✓] Enable Network Adapter
• Attached to: [NAT Network ▼]
• Name: [NatNetwork ▼]

However, if I do so my virtual machine is still able to reach my modem, the host system and any other device in the LAN.

Is my setup hypothesis correct? If not, what I'm missing?

Update: some more info

Host

• Ubuntu Desktop 20.04 LTS
• root permissions available
• WiFi connected
• ethernet hardware available and working but not usable (because of the position of the PC)

Guest

• Ubuntu Server 20.04 LTS

If other info are needed, please ask for them.