Cannot remove 159 devices unconnected

I've just got a new COX modem.

But it turned out this is OLD one with 159 devices from the previous owner.

While I can remove one by one with few clicks ... but cannot remove all 159 at one time.

I removed a couple and gave up

Talked to Tech support and no solution.

Reset did not help.

it still shows up 159 devices unconnected