Not able to write CVs to Digitrax decoder

I bought a used N scale Life-Like GP20 at a train show this weekend. It's the later model with the split metal frame and no native DCC support, but the previous owner added a Digitrax decoder to it. I can't determine the model of the decoder. It works fine when I operate it, but the problem is that I can't write any addresses to it. I've tried a factory reset, but since the decoder isn't accepting any CV changes it's had no effect. What things should I look out for to determine why the decoder is stuck in "read-only" mode?

I'm using a Bachmann Dynamis (by ESU) DCC system. I am able to write CVs to all of my other DCC engines, including another Digitrax-equipped model, so I don't think that is the problem.