Add 'lost_password_link' => 'https://ucs.domain.tld/univention/self-service/#passwordreset' to the config.php. From the host something like this should do (i didn’t doublecheck):
univention-app shell nextcloud sudo -u www-data php /var/www/html/occ config:system:set lost_password_link --value="https://ucs.domain.tld/univention/self-service/#passwordreset"