@ Barby_93 

Hello, thank you for your detailed post, you could try some suggestions here for powerline unstable issue: 

1. Please connect a computer directly to the main router by cable and confirm the router is stable. 
2. Turn off Power-saving mode on powerline devices：
 https://www.tp-link.com/support/faq/2018/
3. Put powerline units into the same room and test stability
 

If the issue persists, can you help confirm some details for the issue: 
1.May I know House Led/LED Status on both devices when the issue happens? 

Do you the wireless or wireless is unstable every few mins, will it work again automatically or do you need to reboot poweline unit to make it work? 
2. Please download tpPLC Utility and check its firmware 2 powerline devices and let us know： https://www.tp-link.com/support/download/tl-wpa4221-kit/#Utility