The internet service provider I deal with does not use VLAN ID. 

  @nazmiuyar 

Hi,

On a TP-Link router, if you don't require the VLAN ID, then you need to turn this feature off (i.e. untick the tick box).

  @woozle 

Hello, there is no box selection to turn off the internet vlan id. I solved the problem like this: I selected "bridge" in the connection configuration. It only connected when I entered the wan username and password. When we select the "private" option at the connection stage, it asks us to enter a number for the internet vlan id and cannot make the pppoe connection.