As I know, only firmware of C7 V2 for Unifi users supports guest network... If you just purchase it, you may contact the shop where you bought it to change the WDR3600 to C7 V2 as the guest network...

As to WR702N, HW V1.2 equals V1... WR702N is working as a wireless router? Reset your chromecast and try to reocnfigure it. If the problem continues, can you provide a screenshot of the error?

Any anti-virus software or firewall on your clients blocks it? It should not be a router issue.

What's the hardware version of your W8968? The internet server is using port 80? What's the IP address of it? On web interface of W8968, you can go to NAT-virtual servers to configure it.

If it is secured with WEP, the password cannot be 28 digits... Can you login web interface of your main router to have a check? Is it Hexadecimal format or ASCII Format? The key type is 64-bit,...

Read the article below. It may give you some help. http://www.tp-link.com/en/faq-409.html