Why can't I enter the same VLAN ID value for wired Internet connections in some models?
This is due to a hardware limitation of the built-in switch. Some devices (usually lower-end models) have only one single interface (Ethernet switch) for LAN, WAN, and backup WAN ports, and therefore you cannot use the same VLAN ID number within the same switch.
If you need to use the same VLAN number on the ‘grey’ and ‘blue’ ports on the router, make sure that these ports correspond to different network interfaces, such as GigabitEthernet0 (grey port) and GigabitEthernet1 (blue port).