@boulate:
My idea was : If "4g Modem (192.168.0.1)" can respond to "Pfsense client (on 192.168.0.100)", and if "Pfsense client (on 10.0.2.1 and 10.0.200.2)" can respond to my "Poste de travail local (10.0.1.1)", the it must be a rooting problem only on the "Pfsense client" no ?
It works similar to this.
If your PC in 10.0.1.1/24 sends a packet to the 4G modem, the packet has the source address 10.0.1.1 and the destination IP 192.168.0.1 when it arrives at the modem. Since you have a site to site VPN, the VPN tunnel network itself is irrelevant here. The packets are just routed over that subnet.
So the 4G modem will send its response to 10.0.1.1, but since this address doesn't fit to any of the subnets on its interfaces and it has no special route for this host, it will send the packet to its upstream gateway. Presumable that's the internet provider.
However the modem will response correct to the pfSense clients WAN 192.168.0.100, because this is a subnet connected to its own interface.
So let's do NAT to get it work:
Go to Firewall > NAT > Outbound, if the rule configuration type is set to Automatic set it to Hybrid or manual and hit save.
Then add a new rule with
interface = WAN
source = 10.0.1.0/24
destination = 192.168.0.0/24 (or any if you also want to access internet hosts over the VPN)
translation = interface address
Save it.
This NAT rule will translate the source address in packets coming from 10.0.1.0/24 to the clients WAN address. So your modem will send responses to that back to the client pfSense and this one will send it back over the VPN to the PC 10.0.1.1.