Try to disable UPnP on web interface of your TD-W8980.

The firmware 130506 will work with chromecast. You can try to load that firmware and use it temporarily if it is urgent for you to use chromecast....

You should describe the problem you encountered in details. What's the firmware version of your WR1043ND? Try to troubleshoot as I mentioned here....

As I searched online, beginning with Mac OS X 10.2 ("Jaguar"), Apple has included a PPTP client. http://www.macdevcenter.com/pub/a/mac/2002/12/20/vpn.html

Describe your VLAN configuration on SG108E in details.

"Client Isolation" just isolates wireless computers from each other. 8840T will still work as the main router, right? TD-W8970 has guest network function. Clients connected to guest Wi-Fi will not be...