Take a look at the grid_1 and grind_2 datasets in data/dmrpp.
The problem is likely that the FQN() method of libdap does not include the group parts of the path. If it did, those names/paths could then be passed directly to the HDF5 library.
For DMRs that are built from files with groups, the names are munged. However, an attribute is added that provides the original full name. See below. Unfortunately, two new variables are added (sometimes, all the time?) named `lon` and `lat`.
You can see the tool does not work with the munged name, but does work with the full name (although I used a different variable in the two cases)