The issue could be anywhere on the board. Unfortunately the only option is to send it to a professional to remove the EMF shields and inspect the device under a microscope to see exactly where the issues is. As far as a know though, with the iPhone 6 specifically, most (including myself) will only offer a data recovery service after water damage due to the nature of water damage and the fact that it can be progressive.
For example right now you are stuck in recovery, tomorrow or next week you might loose backlight, and already or down the track things like the camera, reception, touch screen, ect may not be working. The reality is there was almost definitely "other" issues with the device, someone tried to restore it thinking it would fix the corrosion somehow, it didn't, the device got stuck in recovery, and so they sold it.
Who knows what did or didn't work prior to the device getting stuck in recovery, but based on your specific description of the error it sounds like the Baseband CPU is not responding, (which could be an issue at the BB CPU itself or the BB PMIC aka PM8019) and so itunes can't accurately determine exactly what device it is dealing with (although it probably still knows its a iPhone 6)
Also worth mentioning the water damage sensors don't always go red, so if they aren't, it just means they specifically didn't get wet. What water got on the underside of the board/shields is an unknown variable until someone checks it out though.
The water itself isn't the issue, its the impurities in the water that make it conductive/corrosive. Basically drying it out at this point, no matter how much rice you use, will do nothing at all. The impurities from the water have made it somewhere important, and have already caused damage. There's a very high chance solder underneath BGA IC's will need to be replaced, at the least :(