How to allow multiple hostnames to login?

You will need to disable this function discourse/lib/middleware/enforce_hostname.rb at main · discourse/discourse · GitHub

Plugin code would be

after_initialize do

  module ::OverrideEnforceHostname
    def call(env)
      @app.call(env)
    end
  end

  class Middleware::EnforceHostname
    prepend OverrideEnforceHostname
  end

end