But there would be a record of that, wouldn't there? If a tower was not taking calls?
What might be more telling is the human factor. If, for example only, MR said to LE he was one place, but his phone showed he was elsewhere, and Dylan is eventually found near where MR's phone actually traveled, that would be a strong piece of circumstantial evidence, IMO.
Unless Dylan has the DNA of a stranger on his body, things like cell pings and tracking may become more important, if difficult due to the scarcity of towers. In any event, a case like this is bound to be a circumstantial one, as a confession with matching evidence is unlikely.
I don't think he'd be dumb enough to use his phone while he was murdering someone or dumping a body. MOO
ETA: I know we don't know that his phone doesn't have GPS, but we don't know that it does either. MOO