@Loli 

Good day.

May I know how long have you had the power-line adapters?

For the firmware, if you have the EU version, please refer to this link:

https://www.tp-link.com/en/support/download/tl-wpa4220kit/v1/#Firmware

while if you have US version:

https://www.tp-link.com/us/support/download/tl-wpa4220kit/v1/#Firmware

and how to update the firmware：https://www.tp-link.com/support/faq/1614/

please also check the power-line rate between each other;

(PS：since your current firmware is a little old, you might need to update from the bottom one on the link, then to the latest one.)

Thank you!