How to set up Mac mini as LAN gateway

I have a 2018 Mac mini with a Synology RAID connected to it via Ethernet. The Mac mini and several of the Windows VMs back up to the Synology. I want my other Macs on the WiFi LAN (192.168.1.n) to see the Synology (192.168.2.n) and back up to it as well. These are low usage Macs that won't be backing up much.

Right now none of the WiFi LAN systems see the Synology. I connected a Windows 10 laptop to the Ethernet connection and it is getting DHCP errors that I have never seen before in my nearly 40 years of working with Windows:

ipconfig /renew

An error occurred while renewing interface Ethernet : unable to contact your DHCP server. Request timed out.

These are the instructions that I am following, but they don't seem to work. What is missing?

Setting up your 2018 Mac mini as a Gateway between two LANs (Ethernet and WiFi)

Disclaimer: This guide requires advanced networking knowledge and modifying your network settings can potentially disrupt connectivity. Proceed with caution and back up your data before starting. If you're unsure, consider consulting a network administrator for assistance.

Requirements:

• 2018 Mac mini (Sonoma)
• macOS Monterey or later (older versions might require additional configuration)
• Two Ethernet ports (using a USB-to-Ethernet adapter for the second port if needed)
• Two separate routers or access points for each LAN

Instructions:

Configure the Mac mini's Network Settings:

• Go to System Preferences > Network.
• Click the "+" button to add a new service.
• Choose "Create a Location" and name it according to your preference (e.g., "Gateway").

Set up Ethernet Interface:

• Click the "Lock" icon and enter your administrator password.
• Click the "+" button under "Services".
• Select "Ethernet" and click "Create".
• Name the interface (e.g., "LAN1_Ethernet").
• Click "Advanced" and navigate to the "TCP/IP" tab.
• Important: Choose "Manually" from the "Configure IPv4" dropdown menu.
• Enter a unique IP address for the Ethernet interface (e.g., 192.168.1.100).
• Set the "Subnet Mask" to 255.255.255.0.
• Leave the "Router" field blank.
• Click "OK" and then "Apply".

Set up WiFi Interface:

• Click the "+" button under "Services" again.
• Select "Wi-Fi" and click "Create".
• Name the interface (e.g., "LAN2_WiFi").
• Choose your preferred WiFi network from the list and connect.
• Click "Advanced" and navigate to the "TCP/IP" tab.
• Important: Choose "Manually" from the "Configure IPv4" dropdown menu.
• Enter a unique IP address for the WiFi interface (e.g., 192.168.2.100).
• Set the "Subnet Mask" to 255.255.255.0.
• Enter the IP address of your WiFi router (usually found on its label) in the "Router" field.
• Click "OK" and then "Apply".

Enable Internet Sharing:

• Check the box next to your WiFi connection under "Sharing".
• From the "Share your connection from:" dropdown menu, select "Ethernet".
• From the "To computers using:" list, check "Wi-Fi".
• Click "Start Sharing".

Configure Devices on Each LAN:

• Ethernet Devices:
  • Set their network settings to use DHCP (automatic configuration) to obtain an IP address from the Mac mini's Ethernet interface.

• WiFi Devices:
  • Connect them to the WiFi network you selected in step 3.
• Set their network settings to use DHCP to receive an IP address from the Mac mini's shared WiFi connection.

Testing and Verification:

• Verify that devices connected to the Ethernet and WiFi networks can access the internet.
• You can ping devices on each LAN from your Mac terminal using their IP addresses to check connectivity.

Additional Notes:

• This configuration creates two separate subnets. Devices on one LAN will not be directly accessible from devices on the other LAN without additional configuration (e.g., routing rules).
• Consider security implications when connecting separate networks. You might need additional security measures like firewall rules to control traffic flow.
• Modifying routing settings can lead to potential network conflicts. Be cautious and double-check configurations before applying changes.

Alternatives:

• If you only need basic internet sharing, consider using "Internet Sharing" with just the WiFi connection. This would not create separate LANs but allows devices connected to the Mac mini's WiFi to access the internet.
• For more advanced network configurations and security, consider using dedicated gateway devices designed for this purpose.

Remember, this guide provides a basic introduction, and further research or seeking professional help might be necessary for a secure and fully functional gateway setup.

[Re-Titled by Moderator]