Under the AP Configuration - System, set the mesh properties and create a Mesh community:

With mesh role auto, the AP will become a mesh portal if there is an uplink where the AP can get an IP/connectivity; and it will become a mesh point if there is no connectivity. If you have different groups, you can force the mesh role, but make sure the mesh band/metric/community key are the same.
Then under Interfaces - Wired you can create the wired AP profile and link that to the interfaces of your 503H:



Have not really tried this with moving mesh points, but it should work if speeds are not too high (walking speed should be fine).
------------------------------
Herman Robers
------------------------
If you have urgent issues, always contact your Aruba partner, distributor, or Aruba TAC Support. Check
https://www.arubanetworks.com/support-services/contact-support/ for how to contact Aruba TAC. Any opinions expressed here are solely my own and not necessarily that of Hewlett Packard Enterprise or Aruba Networks.
In case your problem is solved, please invest the time to post a follow-up with the information on how you solved it. Others can benefit from that.
------------------------------