Hello,
You mentioned that the SIM card provides a static public IP, so I think the X300B/Collie will be hosted the VPN server.
You can first configure the VPN server on the X300B, WireGuard is recommended, export the VPN profile, enable "Allow Remote Access the LAN Subnet", and connect the LAN clients such as NAS to the LAN port.
Then import the above profile to MT2500/Brume2, and enable VPN client to connect to X300B.
If the MT2500 VPN connection is abnormal, you can test this profile on your phone first to see if it can connect to the X300B VPN server.
Please refer to the following tutorials: