In this case, it should be enough to disable the Masquerading in the WAN zone.
To achieve this, navigate to Network → Firewall → General Settings. Then disable the Masquerading option for the WAN zone.
After disabling it, a firewall restart might be necessary. This can be done by simply restarting the router or navigating to System → CLI.
Login is always root, the password is the same as for WebUI. From there, run the command:
And that should be it!
Let me know if any more information is needed!