From Kodi's docs on the web interface:
Method 1 / guisettings.xml
Edit the guisettings.xml file in the userdata folder and look for the
following settings under <services>
, and make sure they are set to
"true". You'll have to look for them a little bit, because there among
several other settings. Assuming you have not set a password or
changed other default settings, this should work:
<esallinterfaces> - true
<webserver> - true
<zeroconf> - true
Save the file and restart Kodi.
Method 2 / advancedsettings.xml
While using an advancedsettings.xml file might be easier to enable
these services, doing so will hide them from the normal Kodi GUI. You
will have to remove or edit the advancedsettings.xml file to turn
these settings back off, which can be confusing later on if you forgot
you have an advancedsettings.xml file.
Create a plain text file (no rich text formatting, don't use .doc,
etc) and save it as advancedsettings.xml. Make sure that the file
extension is ".xml" and not ".txt" or ".xml.txt". Cut and paste this
into your new plain text file:
<advancedsettings>
<services>
<esallinterfaces>true</esallinterfaces>
<webserver>true</webserver>
<zeroconf>true</zeroconf>
</services>
</advancedsettings>
Save this file in your userdata folder
Note: If you have an existing advancedsettings.xml file, make sure the
tags are between the main tags.
Restart Kodi.