As a quick mitigation, I’d try to go to the Foris GUI, tab LAN and change router IP to something else (and e.g. back later). I tried now on TOS 5 (HBT) and it seemed to change correctly, including the DNS address served over DHCP.
EDIT: on these kinds of changes I think you may need to disconnect the client to trigger new DHCP exchange (at least for my wired one it appeared so).