The unstable OpenWRT 19.07 DHCP & DNS

The latest versions of OpenWRT, i.e. 19.07.1, 19.07.2 and 19.07.03, are unstable with the LAN DHCP and DNS functions. This issues can also be introduced in OpenWRT 18.06 by upgrading to the latest OPKG packages related to DNS or PPP, but the specific mechanism was unknown. 

After upgrading the symptom was: 

1) unable to DHCP new IP addresses to new connected devices in the LAN

2) unable to ping the router in the LAN;

3) unable to resolve IP addresses of WAN websites. 

The router was forced to downgrade to OpenWRT 18.06.8. The downgrade process was painful and requires a fresh reset to the factory default settings fo OpenWRT, which means the WiFi signal was disabled in the first place.