I believe my phone shows the error "Secure Booting Error 1003" because I tried installing TWRP Recovery.
Exactly the twrp for leon in theory should work... I have possible solution for someone to try if they dont mind possibly bricking their device
Sent from my LGL33L using Tapatalk
Here's a link to the TWRP recovery for leon. Hopefully that helps
twrp-2.8.7.0-c50.img 11.1M
https://twrp.me/devices/lgleonlte.html