When using a router to dial out as a PPTP client, there are two ways to build the client, NAT and route.

By using NAT, the router will obtain a PPP IP address from the server and use this IP address to communicate with the server.
All packets from local subnet to the other side of the tunnel will firstly be replaced by this PPP IP address on their source IP addresses, then encapsulated and encrypted (if necessary) by GRE and route to the tunnel. On the server side, is sees a single client.

However, when using route, the client won't use the PPP IP address, it will encapsulates and encrypts the packets from local subnet directly and route to the tunnel. On the server side, it sees a subnet in the client side, therefore, the server and the client should indicate the network address of the other side.

When TP-LINK route, TL-ER6120 act as a PPTP client, it is built in the second way. the tunnel will be established between two subnets and you have to indicate network address on the other side for each device.

Also, you should test the connectivity on a PC behind TL-ER6120 since the source IP of the packets be sent from TL-ER6120 is not belong to the tunnel, that's why you were failed when ping on the router.

I have faced this problem for a long time, and now, it is solved in this way.

Thanks for the explanation..
But what should I change in settings to make it works?

I tried to set other subnet and write static route on Windows server, but nothing...

I used to work with routers from linksys, tp-link, rtc.
Not familiar with configuring VPN on Windows server.
Maybe we can wait for someone who knows much on Windows server.

I am not really familiar with windows servers and not sure whether these steps are feasible or not.

But I think the key is to Tell the windows server to route packet through the VPN interface.

For example

topology: 192.168.1.0/24 ----PPTP server -------------(internet)-----------ER6120--------192.168.100.0/24
Assume ER6120 obtained a static IP address 192.168.137.1 from the PPTP server

on the server, try cmd:

# netsh int ip sh int //check the VPN interface, I don't know whether or not such interface would exists.
# netsh int ip add route 192.168.100.0/24 "VPN interface's name" 192.168.137.1

radvd wrote

# netsh int ip sh int //check the VPN interface, I don't know whether or not such interface would exists.
# netsh int ip add route 192.168.100.0/24 "VPN interface's name" 192.168.137.1

I've add such route, but get error "General failure." when ping address from remote subnet.

I am having same issue here. Has anyone resolved this ?

You're asking a Windows Server question. This is not a Windows Server forum.