The Linksys WRT1900ACS has been updated to the latest version OpenWRT 19.07.6 while the procedure was painful. Here is how to fix it.
Since the OpenWRT 19.07 release, almost every minor releases (except the 19.07.4) has a serious bug that will hinder the DHCP and DNS functions. This bug can be observed every time just after updating to a minor release or sometimes just even by OPKG updating by SSH. The symptom is that the NAT function seems disabled and the internet is down. That's why I have been sticking to the old version 18.06 in 2020.
Accidentally I found a way out: the solution is to switch to boot from another flash boot partition, reboot, and then switch back, reboot again --- that's it!
It seems that rebooting this device even by switching off the hardware power button or unplugging the AC adapter is not a "fresh" reboot --- it still keeps some cache or something in the system and just doesn't flash out. The switching partition method sounds like a deep or clear-up reboot that removes all the incompatibility issues after upgrading. Since this bug was not reported widely and by other device owners, I highly suspect that this bug is specific to the Linksys routers with dual boot partitions.
For detailed instructions for switching boot partitions please refer to Linksys Official Support - How to revert from OpenWRT to Stock Firmware (https://www.linksys.com/us/support-article?articleNum=316324).
