Think that you will not be able to connect the mikrotik outside your office if you have a private ip, maybe if you ask the isp to give you an port fowarding. But not sure if they would like. About fortinet really don't know about it, but some devices show they public ip no matter you have an private ip, show both ip. As it is dinamic and both connected directly to the modem, both should receive the same ip.
Haven't seen this before anywhere in Greece, but first time for everything... right!! The thing is i don't understand why is the Fortigate is giving me the REAL ip and the mikrotik don't !!! The WEIRD thing is like I'm having two dynamic IP's at the same time, the modem "ZTE wind blabla something" has the 22.214.171.124 example and its visible from any whats my ip site!! and when I'm trying to connect from the mikrotiks ip 10.10.10.10 something is only accessible from my internal network. Both of the ip's are coming from different gateways from my ISP. The 126.96.36.199 with is from my wind router zte was a gateway 188.8.131.52 wich is a valid one "I'm working with this sip for over 10 years now".
I will post some photos later on to see my point.