Abi,
Thanks a lot for the solution, upgrading the AP via FTP worked well! It must be said that as you said, the original FW version had to be blocked on my Fortigate so it could not download firmware nor settings/configuration from Ruckus Cloud.
Checked the version via SSH, gives me:
Copyright(C) 2018 Ruckus Wireless, Inc. All Rights Reserved.
** Ruckus R720 Unleashed AP: 3617xxxxxx
Warning: AP is in Unleashed-Managed mode
Current or latest Unleashed: 192.168.x.x / xx:xx:xx:xx:xx
Any configuration changes made in CLI may conflict
with the Unleashed's management and
will cause undefined results.
rkscli: get version
Ruckus R720 Multimedia Hotzone Wireless AP
Version: 200.8.10.3.278
It even joined my existing R710 Unleashed master!
Thank you again for finding the resolution!