The DPF (diesel particulate filter) catches unburned hydrocarbons, then during a regen cycle injects fuel into the exhaust to burn them off and “regenerate” the filter. So if it’s doing frequent regens that suggests you’re getting incomplete combustion in one or more cylinders. Maybe an injector or the high pressure fuel pump is going bad? (Just speculation)

Scottys right that the dealer isn’t the place to go for that. Typical Nissan Service department won’t have a lot of experience with the Cummins diesel motors. I’d take it to an independent diesel shop. Or you could go to a Ram dealer since they sell way more Cummins engines. 

No surprised the dealer can't figure it out cuz most Chrysler dealers are idiots. It obviously has some type of problem and it could be that the cats are starting to wear out. It's a catalytic converter wears out. You'll have to keep trying to regenerate. It's up because it's wearing out. Any good mechanic could pressure test them and stuff. But like I said the guy at the dealers they don't give a crap