A development version with CIDR is available from:
https://www.ads-software.com/plugins/iq-block-country/advanced/
Scroll all the way down and select development version and press download.
You should manually unzip it and FTP/SFTP/SCP the files or whatever you use for your webhosting to you website. Or uninstall the plugin and upload it via the wp-admin GUI (perhaps export your settings first so you can import them later :-))
It accepts 192.168.2.0/24 for IPv4 for instance and 2a03:b0c0:2::/48 for IPv6 and ofcourse you can add multiple ranges next to single ip addresses