Gerri, the firmware of MR3420 is the latest version? L800 can be identified or not? If it can be identified but no internet, you can try to manually put APN, Dial number etc.

USB data card refers to 3G/4G card? What's the model number of it? Is it in compatibility list of W8968? Check if it can be identified. http://www.tp-link.com/en/support/3g-comp-list/?model=TD-W8968...

Ross, W8980 supports NAT loopback indeed. Did you try http://88.247.135.100/ instead of http://88.247.135.100/activecollab/public/index.php in external network?

There is an instruction here. Follow the guide to check your IPv6 settings. http://www.tp-link.com/en/article/?faqid=857

It is a problem related to Vista OS. Change security settings on the router. If it is WPA-PSK/WPA2-PSK, change it to WEP or disable security. Remove the network profile and reconnect. If you don't...

As I know, currently, all TP-LINK powerline adapters are compatible with each other. To guaranty data transfer rate, you can still buy an AV500 powerline adapter.