Do you install the latest firmware? If tplink support cannot fix it, maybe get a replacement.

admin by default. You should be able to find it from the installation guide.

No, it's just powerline adapter not a router. Does not have advanced function like firewall.

The distance will kill some bandwidth. And you can test it in the same room. Check the link rate between two adapters, if it's too low, move the adapter to another socket, move away from high power...

Yes that's the way to do it. This is a strange problem. Contact tplink support to see if they can help you.

Those powerline adapters should be compatible with each other. Did you try pair them with the tpPLC utility?