I've personally found good luck with Discord, because it's very good at having spontaneous conversations and allowing you to keep things posted.

A lot of people I know who use Discord will supplement it with Roll20 (who I don't particularly care for), Fantasy Grounds, or another tabletop as needed. I don't remember much about combat in TDE, so I can't say whether or not you'd need that, but I personally found that having fairly simple maps that you just upload into Discord can be helpful.

Another thing I've been messing around with is Dungeonfog, which is a map editor plus player combination. It's not quite as feature complete in other ways, but it would be a fine supplement to Discord and I prefer its layout and systems to Roll20; the mapping tools are fantastic.