My good friends, thanks to ALL that have replied! I really appreciate your help!
And I have managed to discover what the problem was!! And even proceeded to a solution...
I will leave it here because other people might have the same problem and spend WEEKS making all sorts of tests and reinstalls and resetting whole networks, when the problem is much simpler than we can imagine....
Well, I mentioned that these wifi cameras are cheap chinese generic itens, with no famous or specific brand. they even have no manual. and they connected to an app that is no longer supported on iOS 12, so i had to look for another app that would "recognize" them.
If you look for "p2p wifi camera" or "wifi camera" on App Store and Google Play, you will find tons of apps, many displaying very similar interfaces but different chinese developers.
the problem with my cameras not being accessed over 3G/4G or outside my home wifi network was simply because of the app used.
when I brought my old iPhone 4S with the old app installed and added the cameras, they worked perfectly like before!
My conclusion is that even though these ip cameras mobile apps sometimes share almost identical interfaces and have very similar names, they share different streaming plataforms, and problably neither the 4 or 5 apps that i was testing had access to the same plataform that would share the feed uploaded by the cameras.
After a looooong research on app store i managed to find an app completely identical to the one that doesnt not work on iOS 12/13, tested the cameras and they worked fine.
hope my explanation was clear and hope it helps someone with the same problem!