macOS 10.13.6 does not save DNS servers
I have a MacBook Pro 11,4 running High Sierra 10.13.6.
Recently, I have been unable to connect to the internet over my wi-fi connection.
I did some troubleshooting and determined that the DNS server address in System Preferences>Network>Wi-Fi>Advanced>DNS no longer works. (I think it was my ISP's recommended address.)
I added my router's IP address, which is what works for the other devices in my home. I removed the ISP address, clicked OK, then Apply, and everything seems to be resolved.
The issue is that after a shutdown/restart, only the previous DNS address appears in the Network Preferences. It is as though the settings were never applied.
I have a admin credentials on this laptop, so the settings should be retained.
I've flushed the DNS cache and cleared PRAM, but the system still boots up to only the old DNS settings so I have to manually change them every time I reboot.
Anyone else have this issue or know of a solution? I'm about to wipe everything and roll back to Sierra, where this problem did not exist for me.