Adding UOrb topics for RTPS

Hi, i am currently struggling with adding UORB topics to the published RTPS message stream. I’ve adding a new message (vechicle_gps_position) to the relevant .yaml files. When i run micrortps_agent -t UDP i get:

micrortps_agent   ]	Unexpected topic ID '28' to publish. Please make sure the agent is capable of parsing the message associated to the topic ID '28'

I’ve added the topic to ROS and used ID etc. I generally struggle due to inconsistencies between the documentation and the actual source code. Where can I look, or who can i ask to get some clarity here?