Hi
As far as I am aware when configuring the deployment properties of RDS you can only import certificate in pfx format. If you want HTML5 version of web working then you follow the steps in the guide you are referring step 1. On the RD Connection Broker server, obtain the certificate used for Remote Desktop connections and export it as a .cer file.
I am going test this entire process on a test server first make sure i can automate the entire process then use it in production.