2009 isx dpf/regen issues

Hi all, I’ve got a truck that has me stumped and looking for some help!
I am NOT a mechanic, I can turn a wrench and do simple stuff but the more complex stuff I have a mobile mechanic do, he knows more than I.

I have a 2009 isx with dpf. The truck is a spare, probably less than 5k miles the last year. Recently we have started using the truck more but are having issues with it. We have owned the truck about 3 years. It has just under 900k on it. Engine history is unknown but was told the dpf filter was replaced at 700k. Truck has just over 30k hours.

Recently the dpf light comes on. Had mechanic look at it. He runs a regen and when done no check engine lights on but dpf light is on. Computer says soot level severe. It seems like temps aren’t getting hot enough for the doser to start adding fuel. Truck runs fine otherwise so not sure what to do.

Any suggestions? Or where to start? We don’t want to throw parts at it and pray that it’ll fix it, as that gets real expensive real fast.

Thanks!