I have a DL2100 MC NAS, and I fixed this problem for myself just this past weekend!
I, too, could not reach my NAS remotely with the MC app either. What I did to fix it:
First, I checked in the complete user manual for ideas. Then, from Dashboard, I went into Settings/General to the Cloud Access section and saw that Cloud Service WAS turned On, but below that (in Connection Status) it also showed it was not connected. So, in Cloud Service there is a configuration link, I clicked on it to be sure it was in Auto setting (the default) and saved this config by clicking on Apply,
The Connection Status showed, Connected, so then I checked my remote Access, and it worked again. I checked access via a wi-fi hotspot near me and via my cell plan, both could access the NAS via remote connection and play media files again.
So, problem was caused by lack of connection config. Who knows why, because I never messed in this section. Perhaps a reboot or FW update changed things.
OK, so if this info solves your problem, please be sure to click the indicator button that this problem was solved; for others who see this thread.