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Adopting UniFi Flex Mini or USW-Ultra Switches
Flex mini switch has 5 ports 1st port is PoE IN support 802.3af and it is powered by a 5V, 1A Power Adapter available inside the box which is USB type C.
Adopting a flex mini or USW Ultra switch is different from all other UniFi devices as they doesn't support SSH so you have to adopt them via DHCP option 43 or local discovery L2 method.
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Local controller L2 discovery method (Recommended)
NOTE You should have the local UniFi network application controller software running (Or install UniFi controller software on a PC operating on the local network) in this method and make sure the switch is connected to Network and powered.
Go to settings >> system >> advanced and enable "Network Discovery" and apply the changes.
Go to the devices tab and the Flex mini should be discovered with as status of “Pending Adoption”.
After adopting the device into local UniFi controller software, you can migrate the site to Cloud Unifi. For a site migration you can follow this guide
After migrating the temporary local site to Cloud Unifi you can now move the device to the desired site(if needed). Click on device then go to "settings" and select "Move to New Site". Select the site and click "Move".
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DHCP option 43
Once you powered the switch and connected to the LAN network, you will have to enable DHCP option 43 on your Router LAN network.
Follow this guide to enable DHCP option 43
Once the device has been adopted to the controller, go into Settings >> System >> Advanced, check the "Inform Host" Override option and input your hosted controller hostname (such as example.cloudunifi.com) and apply the changes. Your devices will replace the IP assigned to the inform URL with the hostname of your controller in the inform URL, but it may take 5-10 minutes.