Fatal error: Call to undefined function filter_var()

Call to undefined function filter_var() is not a plugin bug. filter_var is part of PHP itself, and the error means the filter extension is missing or disabled on your server.

FOX uses it to validate IP addresses for Geo IP rules, which is why the error tends to surface when geolocation is switched on.

The fix is on the hosting side. Ask your host to enable the PHP filter extension, or, on a server you control, make sure PHP was not compiled with --disable-filter. Nothing needs to change in WordPress or in the plugin.