Hello@Phil1204,

Please confirm the WPA4220 wireless powerline unit firmware is up to date:

https://www.tp-link.com/support/download/tl-wpa4220/

You could try to reserve an IP address on router for devices connected to powerline unit, and see if that help with stability. 
Turn off Power-saving mode on powerline devices： https://www.tp-link.com/support/faq/2018/
For test purpose,  put powerline units into the same room and test stability
If the above does not help,  may I know House Led/LED Status on both devices when WPA4220 stops working? How about the wired stability on the extender unit? 

May I know how often does the issue happen, and how does the connection recover, do you need to reboot the powerline unit or router or it will start to work by itself?Thank you