EasyApache 4 uses an RPM-based installation method. Thus, you can simply install that PHP module using the following command:
yum install ea-php56-php-iconv
Note that you'd replace "php56" with the specific version of PHP you want to install the module for (e.g. php70, php71). Additionally, you can install this PHP extension by customizing your "Currently Installed Packages" profile using "WHM >> Software >> EasyApache 4" if you prefer to use Web Host Manager instead of the command line.