After correcting the record left by @KurianOfBorg, which is 100% incorrect (You can port forward to an address behind another router/L3 switch no problem) I am locking this one as ancient.
Thanks for your answer, it doesn't seems simple to recover this information from 4G supplier...
So much person who are not technical, like many hedlpdesk in this case, too lame...
Why not try it for a few days and see what happens, it will be cheaper in the long run.
I can't see them charging for a ping to the gateway as its non internet traffic.
@jbredereck I got same problem. I was trying to use IPsec tunnel with routed(VTI) mode. I stuck with same situation that machine behind vpn get incoming packet, but reply was going out through wrong interface (default instead of tunnel).