Hi Jan,
As far as our tests that we have done, only via the SERVICE/RDS port we could send PI and Radiotext commands via UECP only as mentioned, I have looked in the manual and attached a picture of the setup embedded in the transmitter. Before sending UECP, we looked at the RDS sequence that the former station had, and we observed a lot of interesting stuff, such as : Slow Labelling Codes found on Group 1A: Specific application developed by the broadcaster + Unassigned SLC + Language Identification Code + Paging identification + TMC identification + Extended Country Code + EWS channel identification , and a 13B ODA, we have also observed losses of the groups. After a successful attempt to connect, we could send a proper sequence, but we still have group losses.
I was hoping you would have their WINRDS+ software that the manual advises for this exact model with UECP, the other module that they offer advises Magic RDS 3 to be used. Sadly, their support team is not exactly useful, as my colleague contacted them in the past for another issue and they spoke a very broken english, they didn't understand what he wanted from them.