It's really disappointing to hear that you've had issues spanning 10 months without a resolution even after multiple attempts.
I am confident that most of the basic troubleshooting steps have been completed. I'd like to rule out a few things inside of the home just to cover all of our bases.
Is the modem connected directly to the power socket or does it connect via a power bar/surge protector? If it is connected using a power bar/surge protector please bypass it and connect directly to the power socket. If it is connected directly to the socket already, please try using a different power socket. An extension cord may be required to make this work.
Thank you for posting your signal levels. Everything appears to be within the specified range.
Keep us posted if the intermittent disconnect issue is persistent after trying this.