Pimusicbox supports playing music from a Windows share, DLNA is not a Windows share. I don’t think we have any support in the current version for DLNA/upnp servers.
If you want the music stored on mediatomb to play on a particular device, that device must be a DLNA renderer.
Mopidy-dleyna probably does what you want but if I remember correctly it’s not supported in pimusicbox. I can’t remember exactly why off the top of my head.