@cognis0 if the M9s are just sitting in the loft pointing downwards they could be sitting directly over wires/joists, is the loft floored or can you see what's maybe blocking the signal? What work did you get done in the house, could a new source of interference have been added like an inverter for solar panels, etc? Could the electrical cable that the powerline adapter is using have been altered/damaged during the works? Could the tradespeople have moved or damaged the Decos? 

  @cognis0 I'm not sure what you mean by horizontal but the antennae in the M9 will be configured to send and receive signals in a dome pattern on the side with the LED like my highly technical sketch below. Any signals directly under it will be much weaker, so if the unit is in your loft I would suggest flipping the unit upside down to point into the house and the floors below or better yet, install the M9s on the top-floor ceiling by drilling a small hole to pass the power and ethernet cables through from the loft. This would cut out any interference from physical barriers (wood, plaster or metal) and hopefully get away from any EM interference.

  @cognis0 apologies, I just had a proper look at the M9 Plus in a youtube video and apparently it doesn't have any screw mount holes in the base to ceiling mount it. That might not be a great idea if it's not designed for it but with any luck someone from TP-Link will jump in and try to help if repositioning them doesn't help

  @cognis0 glad I could help even a little bit! Just to check, are you in a 2 storey house (ground, first, loft) and are all the Decos in the loft? You might benefit from having a Deco on the ground floor to bridge the connection. It's difficult in stone buildings because the signal can't really travel diagonally, just straight down through the wooden floors like an elevator shaft in your scenario. Fingers crossed you hear back soon