If your ISP provides a manual firmware file (rare for this model), ensure:
Most ISPs, such as , lock these modems so that firmware updates are pushed automatically through the network. sagemcom f-st 5366 firmware download
Disconnect all non-essential devices from the router. Close all heavy applications on your computer (streaming, torrents). You need a stable connection. If your ISP provides a manual firmware file
F_ST_5366_02.21.47.6_O2.bin