PHP support for iconv is required to handle multiple charsets

Hi, I am trying to set up the Tiny Tiny RSS on my Synology NAS, I’ve got PHP 7 installed, and the system has PHP 5.6 installed by default. I download the zip file from Github and unzip the file under the path for my web sever root location.

I was able to open the webpage of install at /tt-rss/install/
but after entering the information and click on test configuration, I got this message show up on the page

PHP support for iconv is required to handle multiple charsets.

What should I do to fix this?

Enable iconv.