"close-session" command RETURNS "failure: unknown router ip"

It is probably because the User Manager (Public IP) is located on another place.
The HotSpot Server (Internal IP) is behind a modem.

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Since now, the User Manager aware of the modem’s address and everything was working just fine (AAA, paypal…)

But the /tool user-manager session close-session command fails probably because the UM cannot access the HotSpot Server
at the known IP address (I’m not sure, I guess it).

Is there anything I can do to get over it? Port forwarding?
TNX