LB2120 Port Forwarding ISP T-Mobile

How to set up port forawing?  When I used Spectrum cable internet my TRENDnet router worked good when port forwarding.  When I tried to setup port forwarding with my LB2120 I get two WAN IP addresses and the port forwarding does not work in bridge or router mode.  I was told that I need a public IP address from my ISP "T-Mobile".  T-Mobile tells me that all their SIM cards are set up for a private IP addresses.  Can anyone help me with this port forwarding issue?  Is there a better or different router model or ISP to get port forwarding to work on a cellular network? Help?