NV1 I traveled several months with cash only and no SIM card for my phone. As long as your EUR bills are pristine you wont have any issues with conversion anywhere. Keep in mind they are very strict on bill quality. Even a tiny ink spot or tiny tear and they wont take it.
If you have a VODAFONE sim card (Belgium has them?) you will be able to roam most likely, at least a few months ago you could. It will be hit or miss on data but just plan for not having it and you'll be fine. Hotel wifi is all I really needed.
Also, keep in mind tourism is alive and well in Russia. Many groups of Chinese tourists are everywhere.
I can't speak for Dagistan or Chechnya but I'd be curious to learn about them for next trip.