Because this is a provider-locked router, you likely cannot download firmware directly from Sagemcom, but must wait for your internet service provider to push it.
Are you looking to or are you interested in open-source development for this device? Request to Add Support for Sagemcom Fast 5866T #13746 Sagemcom Fast 5866t Firmware
Enable (hidden in advanced UI → Diagnostics → Syslog Server). Send logs to a Raspberry Pi. Then you’ll finally see: Because this is a provider-locked router, you likely