I dont know what you can do to solve this problem
the tankless water heaters are nothing but trouble...
and it looks like you plumber is getting his education in your home
did your plumber mention that most of these units
need to be purged every 2 months even when they are working
I have heard it before....
have you considered just putting in a a 50 gallon gas heater
and saveing yourself a ton of greif???
do you still owe the plumber any money???