talli I've taken a look over this extension today. It's not a bug per se, but simply the extension expects a user to click on the forum "signup" link when they arrive.
There is however a rather simple solution. I've begun to add the 3rd party extension Direct Links to the Blomstra stack, which provides a
/signup (and a few others) route to Flarum.
With a small amount of custom development to
fof/doorman (tracked at 26m), I have added support for Direct Links, thereby the URL generated by Doorman when Direct Links is enabled, will now take a new user directly to the signup modal.
Direct Links, and the updated FoF Doorman will be deployed to your community later today. Once this has happened, please enable Direct Links, then you'll have the updated functionality.