SB8200 working but ethernet port never stops blinking

If the Ethernet port is connected to the modem, it is normal for it to blink.

For your information the blinking Ethernet ports typically indicate that data is being transmitted or received over the network. This means there is active communication happening through the Ethernet connection.

If it's normal to blink without ever stopping,  even with no connected devices, then that's a horrible design. That room looks like a dance club

Since you mentioned that the Ethernet port is blinking without any devices connected, please confirm whether there is anything physically connected to the Ethernet port or if it is blinking even without an Ethernet cable plugged in.