On your screenshot in the box it states "Router is behind a NAT. Remote connection might...."
If you have a NAT "upstream" (your ISP) that you cannot control, inbound connections might not be possible, nothing related to 6.x or 7.x
Your rule is exactly like mine, I run 7.5 at the moment.
IP Cloud only takes care of the "dynamic DNS" part, nothing more.
It dosen't metter. my Rb750 Gr3 is also behaind the and IP Cloud is working.
Suggest any other this if any
The rule you show is only related to the IP-cloud part, nothing to do with DNAT for making the actual inbound connection to a server/service you host.
So, if you resolve your XXXXX.mynetname.net
how does it compare to any of the IP-addresses on your box ?
If you don't have any hits on that "input" UDP/15252 rule and you are sure of the config you should talk to you ISP.