Hi,

From the picture, we can see that the powerline speed between the main unit the lounge unit shows "?", that means the the speed may be slow, it is suggested to change another socket to plug it and test whether it works properly.

And what is the light status of the lounge unit when the connection lost?

Meanwhile, cause only this lounge unit have the unstable connection issue, you can exchange the positions between this unit and others to do a comparison analysis. 

Besides, you can refer to the instruction below to narrow down the issue first. 

https://www.tp-link.com/support/faq/2230/

For your last question, you can use tpPLC utility to change some settings of the powerline adapter, like change wifi settings, update the firmware and so on; you can refer to 

https://www.tp-link.com/support/download/tl-wpa9610-kit/#FAQs

https://static.tp-link.com/1910012196_tpPLC%20Utility_UG.pdf

May it help and have a nice day.