Smartphone multiplayer - NAT problem?

I have read several articles and heard that there is a problem if I want to use Unity built-in networking for smartphone multiplayer.

Some say it is possible, some say it is not.

The only thing I want is to make 1 vs 1 multiplayer using iOS and Android and I want to avoid using middleware such as Photon, since this game must not be limited (unlimited online players).

So should I use Unity built-in networking or is there any other way?

I have made an android multiplayer game with the built in unity networking. It worked just fine for me.

Try mobile networking (not WiFi).