Sorry if I'm a bit late with this response, but I just got (and fixed) this issue today, so just in case someone else gets this problem I think it's worth sharing (even though the internet is very sensitive about "old" threads")
This is a solution that I found in this thread:
After I updated my MacBook Pro 15" from 2015 to Mojave (macOS 10.14), I suddently couldn't use the wifi. It said that I didn't have any wifi hardware installed.
In addition to not having an intenet connection, the machine also became ridiculously slow and unresponsive. I then used one of my other computers to search for
answers, only to find that a lot of people were struggling with the same issues. The following "fix" told me to use a sudo command to locate the file, but this
didn't find it for me, so I did the following that did work (although the sudo command may work for others):
/Volumes/Macintosh HD, got to /etc/ and rename/delete the sysctl.conf file, then reboot!
The "etc" is a hidden folder that lies directly under "Macintosh HD", but you can easily find it by selecting "Go to folder" in the finder menu,
and then search for "/etc/". In the "etc" folder you may (if you are having this problem) find the file "sysctl.conf". You must either delete or
rename this file (I deleted it). Then the final step is to restart the Mac! ... And hopefully this solves your problem (like it did for me)!