matt on 22 Dec, 2019 11:52 PM
Sorry for the delayed reply here. First, I want to note that this is not a supported use for the CalTopo WMS/WMTS endpoints; you'd be allowed to enter your own endpoint into ATAK to load map tiles on the fly, but generating large-scale offline datasets, particularly if you intend to redistribute them to others, is not allowed.
I can't provide any advice wrt MOBAC, but I imagine ATAK is probably looking for a WMTS-style (z/x/y) endpoint rather than a WMS one. For that, you should try (also note most layer have a max zoom of 16: