I can’t find anything about the Somfy Mestriaa+50 io. However, it seems like the Somfy IO products do not use the RTS motors. For example, when I check this and look at the “Technical characteristics” then it mentions “Radio Frequency : io 868 MHz”.
If this matches yours, then you cannot control them using the RFXtrx433XL. The RFXtrx can only control the RTS motors which use a frequency of 433.42 MHz.
You can control the IO motors in openHAB using a Somfy Tahoma or Somfy Conexoon and the Tahoma binding.
I don’t have any experience with the Somfy IO products. All my rollershutters use the RTS motors. A “how to” for using the Somfy RTS rollershutters and RFXtrx with openHAB is available here.