When at my mom's when I'm in the back room, I can't get cell service nor can I send a text. Nothing goes through. Not even 3G. Nothing.
I have to move around the house to find a bar for cell service which is sketchy at best and the same with texts. They will queue up and when I go out front, sometimes they will download and I will get them. But I've sat out front and text and have them not go through and it tells me to try again.
As far as Marks actions/reactions when he got home and discovered Dylan wasn't there. The kid is 13, he's not a baby. He's got a mind of his own. He could have been a number of places, there was no reason for Mark to think right away that Dylan was in danger or missing. That's not the first thing he would think of. Why should he?? His son has friends there, there is a creek out back, there is a lake down the way. There are too many things he could have been doing. So maybe he wasn't concerned to start with.
Then after a time, he texts him or tries to call him and he gets no answer or the calls don't go through. So he starts to look around to see if he can find him. It's still not an emergency situation at that point. Why should it be?
Later, he drives to his friend's in Vallecito, finds he's not there. This is probably when he starts getting concerned, because his friend didn't see him or heard from him. So he drives to Bayfield hoping against hope that Dylan found his own way there. This is when he realizes that he has an emergency situation. This is when the panic sets in, when he's not anywhere he thought he would be. JMO
And Mark may NOT have been the last to see Dylan. Someone, the person who has him was.