This switch does not have a web interface. You must adopt it via the UniFi Controller software.
See https://dl.ubnt.com/qsg... If you can't find the switch, I would connect a PC directly to the switch via cable and run the controller software on the PC.
I don't use the software but a cloud key because I have several other Ubiquiti components.
I think the only other way to access the switch is via SSH.