In diesellandia as I am with my F25 X3 30D, I use Penrite diesel fuel biocide treatment, it kills the diesel bug and keeps your fuel lines and injectors clean. I can’t remember the actual concentration, about 30ml each time I fill up is a scratch over the recommended running dose. On the side of the container are instructions for quantities for treating deep clean and also infestations. Wouldn’t hurt to try a good dose of that through yours. I certainly noticed a difference on mine when I first treated it.
have you established with a scanner when the last DPF Regen was? I’d guess you’re getting it up to temp and sustained temp with a fully charged battery for it to kickoff when required. If there’s anything preventing the regen in terms of operating state prerequisites, you might need to get that taken care of.
Also, check for recall - there’s a recall globally for n57 - something to do with coolant and DPF, I forget the details. Continental have the parts to do mine on back order.
HTH