Solved! Brother printer not connecting to WiFi (Error TS-04)
First off, make sure you try other basic troubleshooting steps first if you cannot connect to a wifi router/network.
Two things to note:
Brother printers cannot connect to a 5GHz wifi signal. More info here.
Brother printers only support certain WiFi authentication methods/encryption methods [Important!] More info here (TS-04 section).
Issue: Although my router was set to 'WPA2-Personal' with 'AES', my Brother printer was detecting it as 'Open' with TKIP+AES encryption. I know my printer was doing this because it automatically prints a WLAN Report when it fails to connect to a router on setup. See the first link for info on WLAN reports. So 'Open' with TKIP+AES encryption is not on the supported authentication/encryption list which was causing my TS-04 error.
Solution: I went into my router's settings and changed the authentication method to 'WPA2/WPA3-Personal'. I left the encryption method as 'AES'. My printer was successfully able to connect to my WiFi router now. The new WLAN report shows that the printer detects 'WPA/WPA2-PSK' as my authentication method. Not sure why it's doing this but I hope this gets fixed in a future printer firmware update.
Tip: Some Brother printers have a 'network reset'. This clears out all old wifi settings. I recommend doing this at least once. Look at your manual to see how this function is accessed on your particular printer model.