First, you need to make sure that your phone has a public IP so that it's accessible from the internet (which is usually not the case). Otherwise, port forwarding doesn't make sense. See How to connect to Android over 3G/4G public IP?
Using an Android phone as a USB modem + router is rather simple. That's what we do by turning on the WiFi hotspot or USB tethering. On rooted phones, port forwarding works by adding DNAT firewall rules. On non-rooted phones, you can use
netcat or apps like Fwd (ports under 1024 cannot be forwarded though). For details, see How to port-forward internet using Android Hotspot to PC?
However, transparently forwarding all traffic (and not specific ports) to a Linux server - using Android as USB modem only - is complex (it won't work without root access). It works by manipulating network routing (tables and RPDB) on Android and optionally on a connected host. See How to assign Mobile Data's public IP to a host connected on a hotspot?