Dear @JohanV,

I assume I need to configure port 15 as tagged with PVID 100 and should be combined with port 13 what should be untagged.

But I don't get it working.

So the purpose is to get Port15 and Port13 to communicate with the Router over VLAN 100.

I assume the port13 will be connected to a non-VLAN aware device, right?

If the Port15 on the Router is configured with VLAN 100 tagged, then you need to configure VLAN on the switch as the following:

1) Create VLAN100, select Port15 as Tagged, Port13 as Untagged; 

2) Configure the PVID for Port13 as 100, leave others as the default value; 

3) Test the connection.

Here is the configuration guide of VLAN for your reference. https://www.tp-link.com/support/faq/788/