After 3 months of struggling to centralize the maintainance of 20+ Synology devices with CMS app i have found the solution that Synology support explained me to be a bug.
Solution is simple: go to External Access – Advanced – and put your hostname / ip inside.
In max 10minutes your device will be confirmed.
Solution i posted originally here and it is reported to be working for others.
This solution was created with the help of sysadmin from www.chroot.ro