did you tried to install a phase coupler for powerline improvement
https://www.kemo-electronic.de/en/House/Home/M091N-Phase-Cou...
[ my public key: https://keybase.io/pelim; my proof: https://keybase.io/pelim/sigs/o04o5psu_VBQXOX1Ih3WR2oZiJWZwi4eEaqqozg6MQ4 ]