call your ISP first to check the adsl line. If the adsl line is okay, call tp-link for a RMA.

Topology: Primary router ----------- wan.Vg3631.lan -------PC On the VG3631, configure Dynamic IP for the WAN settings. Then change the LAN IP to 192.168.137.1, or something else than 192.168.178.x....

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...

That is really strange. I think you may check the firewall and PPTP settings on the remote clients first. May also tracert the remote host from the LAN host to check the route. On a LAN host, run...

can the remote hosts ping the LAN hosts in reverse?

Edit, Sorry I miss your point just now. remote hosts ------------(internet)---------------600VPN -------LAN hosts If the remote hosts can access the LAN hosts, the route should be no problem. Check...