fig

For anyone following along using Rails 6.0 - an extra step is required.

If you seeing an error like:

Blocked host: a620ba29.ngrok.io
To allow requests to a620ba29.ngrok.io, add the following to your environment configuration:
config.hosts << "a620ba29.ngrok.io"

Open your config/environments/development.rb file and add:

# Whitelist ngrok connections to development enviroment.
config.hosts << /[a-z0-9]+\.ngrok\.io/

This will whitelist the ngrok subdomain to access your rails host.

credit: Mike Rogers