config file used to be called main.inc.php in previous versions so in Hestia repo the file checks OS version and renames it to config.inc.php if using Ubuntu 18.04 or Ubuntu 20.04.
The file you need to modify is: /etc/roundcube/config.inc.php
If you are referring to:
/usr/local/hestia/install/deb/roundcube/main.inc.php
This is the location of Hestia configuration file which is used to replace the default Roundcube config /etc/roundube/config.inc.php or /etc/roundcube/main.inc.php in earlier versions.