@Syloo 

Thank you for reach out to us. 

Please confirm your house extension (a garage/ barn etc.) and the main building is under the same electrical meter, and powerline device are plugged into wall outlets.   

You could try a different wall socket for the powerline adapters & try some tips here to exclude some factors which could affect powerline environment. 

What can I do if my powerline rate is very low?   

If still the same, disconnection could be due to the weak power line signal over long distance to the  garden, you could consider other types of network device to extend wifi to the outdoor. To extend Wi-Fi signal to an open area like garden, an Outdoor Access Point like Outdoor EAP may work for you, please start a thread in our Business sub-forum so moderator or users could provide suggestions there.