tcs wrote

But I can ping between different VLAN.

The TL-ER6120 is a router, so in port VLAN mode it just adds VLAN tags to the outgoing Ethernet packets making the port VLAN trunk ports. For incoming VLAN-tagged Ethernet packets it just routes them.

It does not make much sense to have two (or three) different VLANs with untagged ports if your device just has two (or three) physical LAN ports. What would you want to achieve with such a setup? The only useful achievement for which VLANs are needed on a TL-ER6120 is to exchange data between different VLANs with the TL-ER6120 router over one or two physical connections, so that separate broadcast domains can be created. For this to work, you also have to assign different subnet IPs. Using VLANs therefore provide a mapping of VLAN IDs to different subnets, which then are undergoing routing (that's what a router is supposed to do).

Therefore, to use the VLAN capabilities of the TL-ER6120, you need either a VLAN-capable switch or servers which support VLAN trunk ports:

Mr.J wrote

Moreover, one thing you should know, the VLAN function in ER6120 v1 is not the same with 802.1Q VLAN. They are different.

Hm. So how is a switch using 802.1Q VLANs supposed to work with the topology such as shown in #4 above?