Yes indeed, sounds like you need to pair the extra one into the existing network. Unplug the extra one. On any one of the live ones, press the button for about a second. Now plug the extra one in,...

What you're trying to do is called link aggregation, and the PLC adapters don't support this. Besides, you'll learn that only on an exceptionally good and short PLC connection will the PLC speed...

Exactly - that is why as a device vendor, one should place the vendor and model information into the "vendor" and "model" data fields of the firmware, not the "unit name" field.

Try a factory reset on the adapters first. Once someone renamed the adapters, using any other powerline management tool but TP-Link's own, the tpPLC utility cannot identify its own adapters anymore....

That or the LED function has been configured to "always off" before. Has that kit been unpacked before, any hints on the packaging? Whatever you do, try a full reset on both units (very long button...

It is IPv6 with IPv4 still supported as an extra. That's not the point though. Every single Windows PC uses IPv6 by default, and it's way beyond normal home users to dabble with that. The main point...