Sounds like protocol 41 might be an issue...does your ISP block it?So I ran a wireshark and all the packets with ipv6 addresses I saw are dns queries. so you might be correct that the issue being on the ISP side. but I have been informed it isn't. I'll double check to confirm.
You can run wireshark to see if you're getting any incoming packets.
You might try seeing if you can configure the tunnel directly on the TP Link router.
6in4 has no issue with NAT as long as the router knows what's going on and is able to pass the traffic without bothering it.
In which case, an obvious suggestion is: you might try putting the windows machine in the DMZ.Thank you for all these inputs. I tried putting the machine in the DMZ. no luck so far. besides my router is not a high-end one. I'll first make sure that the ISP side is in the clear. then I can find a way to configure the tunnel on the router.
I'll update soon. Thanks a lot again.