The national tech group has many specific channels on Mattermost to support with a specific application:
If you have a question that doesn't fit in any of these channels, or if you don't get a timely reply, send a message in the generic tech channel
If you still don't get a reply on that channel, or if you don't have access to Mattermost, then you can send a private message to some of the people active in those channels, or email tech@extinctionrebellion.nl