As mentioned in Part 1, literally go to manageengine.com/opmanager/download > click "Free License" > enter your email. Within 5 minutes, you receive a permanent key for 25 devices. For a small office, this is 100% sufficient.
If you have multiple legacy keys (e.g., two 250-device keys), you can request a license consolidation from ManageEngine support to create a single 500-device key. Manageengine Opmanager License Key
ManageEngine offers a genuine, permanent free license for small networks. As mentioned in Part 1, literally go to manageengine