That does not depend on RSSM but on your server setup. The php codes usually display in .php files only, but .html or .htm files can be made to allow php codes inside by making some changes on server settings.
If you want to execute dynamic PHP code on otherwise static .html and .htm webpages, you need to enable PHP parsing on this type of webpage. To do this, simply add the following line to your .htaccess file and then you can include PHP code directly in the source code of your .html and .htm webpages. Make sure that you embed all PHP code inside opening and closing PHP tags ().
AddType application/x-httpd-php .html .htm
If you do not have a .htacess file, just create it with only that line inside. You can also ask help fom host support.