I have the EZ Pass in Florida. Using the portable version with the suction cups. Mine is mounted to the left of the rear view mirror per the illustration posted by ryzvy. No problems with both fast (highway toll crossing) and slow (airport and other EZPass accessible readers) reading.Thanks very much - that picture helps. Do you have a heated windshield? I do not have one, and my cell reception has no issues with an iPhone 7, but I only use it for voice calls.
I will go to the main EZPass store in Raleigh, NC and I understand they have instruments which determines the best placement. If still a problem, I may have to get an external transponder. Will keep the forum updated.