Unifi uses option 43 so the switches and AccessPoints can find its controller if it is not present on the same L2 network.

Option 43 should be filled out with HEX digits.

Example of the HEX value for a controller with the ip address of 192.168.11.4

0104C0A80B04

The 0104 is the default for Unifi and will always be the beginning of the HEX value
The C0A80B04 is the IP address written in HEX

You can use this link for converting to HEX value: https://www.browserling.com/tools/ip-to-hex

C0A80B04 -> 192.168.11.4

When this is added to the DHCP Scope the accesspoints will contact the unifi controller directly