Change the channel on your existing router to 1/6/11 and then try again.

Read this instruction.Maybe you can check if there is any thing missing: http://www.tp-link.com/en/article/?faqid=244 What is more, it seems it is the compatible problem but not quite sure.

I do not think so because the VLan is based on the Physical LAN port.

Can you use the WAN ip address to view the LAN server because the router supports the lookback. Check whether it is the problem of the DDNS or the lookback function.

They said these will not support Ipv6 at the moment.

TL-WDR4300 have a function called WDS and can connect to the existing WIFI. However, the WDS function requires the existing router or WIFI hot spots support the WDS function. Here is a reference from...