Problem is that you didn't apply the stepdown.
Cat III to Cat I ILS to non-precision to visual. You simply went from one non-precision approach to another. You should have planned for visual weather minima.
If you need an alternate, you MUST have a usable one at the planning/dispatch stage. You can always revise that later when enroute in the light of more up-to-date weather reports, ILS's becoming serviceable, etc.
You didn't have any protection from weather at DTM becoming worse. There was some redundancy in the equipment, but that's only one reason for the way the rules are formulated. You missed the other.