Data traffic through 4G usb dongle and router


I have a modem router connected to PC via ethernet cable. The PC can use a backup connectivity through an usb dongle (with a SIM card) inserted in the router: when the DSL network goes down, the dongle wakes up, attaching to 4G network and then the internet connection works again.
Theese devices are located in a simulated network and also the SIM is enabled to work only under this network: it doesn’t work in the “real” 4G network. At the same time, there are some servers in this network too.

The target is to do some data traffic using the dongle connection. So I do download data traffic sending an iperf udp session from the server to the dongle IP address or a ftp data session connecting to the server through filezilla.

The problem is that theese solutions don’t work. If I do this with a smartphone or with a dongle directly connected to PC or a CPE, it works. But with this configuration (router+dongle) there is something wrong. Is it possible that there are some options to set up? Port forwarding?

PS: if I insert a real Sim card in the dongle, there is internet connection and i can download files.