I had the exact same issue. Some days were better than others, but usually the wifi was unusable.
Android auto worked sometimes, but it was almost unusable.
***What worked for me:***
# Removed the back-cover.
# Gave all(6 in total) back-cover connectors a slight bend upwards very carefully to give better contact towards the back cover
# Using isopropanol: cleaned the connectors on the phone and the metal surfaces on the back cover
# Reassembled
Directly after startup I got strong WiFi signal, I'm so happy I even tried!
I hope this helps someone out there!