Remove the SIM card to ensure it doesn’t communicate with a cellular carrier. Then go into the settings for your specific WiFi network, configure IP address manually, and remove the entry for “Router” to prevent it from talking to the Internet

You jest, but we use Launch Darkly at work, and it’s the shit. Way better than our previous home grown solution. Everyone made the same joke at first, but the value is real

Used HP ProLiant. It’s nearly 10 years old, but has 16 cores 64GB of RAM, and is just under $150 with free shipping

https://www.ebay.com/itm/235286275608