FTP with Aleos/Airlink (RV50x and GX400)

We are having some problems with sending data to a FTP server from a PC behind a RV50x or GX400 router.
The router is used for remote access to the PC through the internet and sms messaging using a public ip address from Spark sim card here in NZ.
Pretty sure they don’t do static ip addressing on their mobile sim cards.

Using either active or passive mode with FTP doesn’t seem to work. We can connect to the FTP server but cant send or receive any data. So seems like the data ports aren’t quite opening or the firewall is blocking them in the router. We have managed to get it to work, only by turning the DMZ on in the router, which is not a preferred solution.

Has anyone used FTP with these routers? or have a document using FTP with these routers.