If you got a Fixed IP SIM card with a static public IP address, you should have received a specific APN from your Mobile Network Operator. In such a scenario, you should manually set up that APN on your router (as was previously commented by OleksandrL).
To manually configure the SIM card APN on your RUT240 according to firmware version RUT2_R_00.07.01.4, follow the steps below:
In your RUT240 WebUI interface, look at the left tabs on the screen and select "Network"; then press the "Interfaces" option, choose the Mobile interface, and click on the edit button (pencil icon). A new window will prompt your WebUI interface. Switch the "Auto APN" toggle to the off position, enter the APN info specified by your Mobile Network Operator, and save it. If you have followed these steps, you should get a persistent static public IP address available for your use.
In case your operator is dynamically assigning Public IP addresses to your SIM card, you could configure the DDNS feature on your RUT 240. This feature will allow you to get remote access to your network even though your public IP address eventually changes. For more information about how to configure the DDNS feature, please refer to the links below:
I hope this information helps with your query. I look forward to reading your comments.