I upgraded several RF820 routers to the 1.40 version recently and since that time, it appears that the DHCP server function stopped working. I restarted the routers and a few hours later we can't get an IP address from the router.

After you upgrade the firmware to 1.40, remove all the DHCP server scope and range and recreate the scope and range with a lease time of 4 or 8 hours.