Configure/Override EU Cookie Compliance
The settings form is hard-wired to only display the config field on the form if one of the following is installed: the Smart IP module, GeoAPI module, or the PHP geoip extension. Outside that you'll have to bypass the config form; if you're version controlling your config, make the change in the eu_cookie_compliance.settings.yml file; or use drush cset to make the change directly.
The hooks are documented in eu_cookie_compliance.api.php. You're wanting to use hook_eu_cookie_compliance_geoip_match_alter() that lets you alter the results from the eu_cookie_compliance_user_in_eu() function. The parameter $geoip_match is a keyed-array with country &amp; in_eu keys that drive the logic.