Diagnosis
The printer's wireless setup process is finicky, especially with certain router settings like WPA3 or 5GHz-only bands. It often fails if the network name (SSID) or password has special characters.
How to Fix It
First, try the WPS method if your router has a WPS button. Press the WPS button on your router, then within 2 minutes, press and hold the Wi-Fi button on the printer's control panel until the network light flashes. It should connect. If not, use a USB cable for the initial setup to configure the wireless through the Brother software on your computer, which is more reliable.
If That Didn't Work
Temporarily change your router's settings. Disable WPA3 and use WPA2 only. If you have a dual-band network (2.4GHz and 5GHz), make sure the printer is trying to connect to the 2.4GHz band—this printer doesn't support 5GHz. You might need to temporarily create a simpler network name and password without spaces or symbols.
Heads Up
After setup, you can revert your router to your preferred security settings. The connection should persist. If it drops later, a power cycle of both the router and printer usually sorts it out.