Strip voltage does not really matter to WLED since it only outputs the data channel signal at the microcontroller’s output voltage (~3.3V). Using a spare LED pixel or level shifter to boost it to 5V for the data channel is a relatively common practice. Most 12 or 24V addressable LED strips accept (require) a 5V data signal. There may be some edge cases out there that would not work directly with WLED or have claimed direct support.
As others have said, hook it up in a controlled environment - on the workbench in the garage or someplace like that. Tell WLED which pin to use and see if the first 30 LEDs turn on.
Tips:
Be careful with the 12V, keeping it away from the ESP controller pins.
Be careful and tie the 0V (ground or common) terminals of the 12V and 5V supplies together (called bonding them together). This makes sure the data signal is correctly interpreted by the LED strip, prevents blowing up the first LED of the strip, and eliminates one of the causes for LED flickering.