Put those two powerline units in the same room or in the same power strip to test the connection. If problem disappears, then the cause could be your electric circuit environment. Move it to another...

I cannot see how this happen. One 9020 connects to your router via Ethernet, other 9020 connects to your computer, you can access internet on the computer, but other wireless devices lose internet...

The login username and password are both admin. You may login to check the wireless network name and password of both extenders. Maybe one of the units still has its own wireless settings, so you can...

The powerline is like a simple switch, it will pass 802.1q VLAN frames, will not block it.

You are using the tpPLC utility? Make sure the Ethernet led is lit when the adaptor connects to your computer. And see if you can disable firewall and antivirus on the computer, run the utility...

I don't see the MR6400 can work as VPN server or client. So you have a software on a mac to connect to VPN on other side, right? Is it only the mac cannot access internet? If that's the case, you may...