The firmware is controlled by your Internet Service Provider (ISP), not Nokia directly. The ISP customizes the firmware for their network bands, authentication protocols (SIM cards), and UI.
You will almost certainly not find an official public download for the firmware. nokia fastmile 5g gateway 3.1 firmware download
Here is why, what you should do instead, and how to check if you are already up to date. Unlike a standard retail router (like a Netgear or ASUS), the Nokia FastMile 5G Gateway 3.1 is a carrier-provisioned device . It is technically a “CPE” (Customer Premises Equipment). The firmware is controlled by your Internet Service