Most cellular internet plans do not give you a public WAN IP. In order to access services behind such a connection you usually have to add an additional service to the plan/account. What each carrier calls this varies but generally you want to ask for 'public IP service', 'static IP service' or 'VPN service'. One you get a rep who understands what you want, for an additional $10-15 a month they'll add it to your account and then all you need to do is forward the ports. Static IP service is usually a bit more, but the IP will never change and you won't need a DDNS account.