Can't assign manual IPs — router displays login prompt, changes unsaved

I'm running 378.56_0 and am trying to assign manual IP addresses after resetting my router to defaults. My problem: after adding some manual IPs in the LAN > DHCP Server tab, when I click the Apply button, the login popup reappears and, after logging in, I see that my changes were not saved.

I tried in both Chrome and Firefox. What can I do?