I don't know why the computer is doing that. You could try resetting the NVRAM by cold booting while holding down the command-option-p-r key combination until you hear the boot chime a third time.
If you have to use a fixed IP, try to pick one well away from the IP's that the DCHP server seems to be using, perhaps 192.168.1.250
[doublepost=1483123609][/doublepost]You can try this suggestion from
https://www.cnet.com/news/fix-self-assigned-ip-addresses-in-os-x/
To reset the firewall, go to the /Macintosh HD/Library/Preferences/ folder and remove the file called "com.apple.alf.plist," and then restart your computer. After the system boots, you may be prompted to allow incoming connections to numerous programs and services, so accept these for now (you can always go to the Firewall settings and deny or remove entries later on) and then try connecting to the network again. While configuration changes from migrating or restoring a system can lead to this problem, at other times major system crashes or power outages can do the same.
