The latest firmware for the LT18 AX1800 router is always posted here from our own servers: https://thewirelesshaven.com/lt18-latest-firmware
LT18 DataSheet
LT18 Quick Install Guide
Router Product Page
The LT18 AX1800 uses the Quectel EG18 modem inside. Here is the official Quectel EG18 AT Commands manual.
Alternatively you can access firmware and documentation directly from Cudy here: https://www.cudy.com/4gxz