What's the distance between those two powerline? Install tpPLC utility to check the rate. If it's too slow, then source could be distance, or other appliances around. Suggestions:...

Check the label on the back. Both are version 2 HW and US?

I thought QoS is disabled by default. Isn't it? 350-450 mbps is just the powerline rate, it's different from the internet speed. And the powerline rate may not be enough. If you have AV500/600,...

It's odd. Update firmware of your powerline, contact tplink support if no luck.

How do you see they cancle TL-PA9020P? I see from the website that that model is still active.

That's strange. Did you set any filtering on your router that only allow the old PS4 to connect? If not, better contact tplink support again to get to the bottom of it.