Possible to disable "disconnected, extensive data loss" ?

Dear all,

I have a device that shall have access to the internal network (being connected by WLan), but no access to the WAN (Internet).

Therefore, I added a firewall roule for the device’s Mac adress that prevents forwarding its packets to WAN - which works nicely.

However, now this device gets dropped whenever it tries to access the internet (WAN):

“disconnected, extensive data loss”

Now, of course that’s true, but I don’t want that this device gets its connection dropped.

Can I disable this automatic disconnect for this device somehow?
Is there another, better, solution?

Thank you, Hannes