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"
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