There are other third party extension managers you could use... those are just the two most popular.
There is this one: http://zxpinstaller.com/
and this one: https://install.anastasiy.com/
You can also install command line following the Adobe instructions here: https://theblog.adobe.com/installing-extensions-for-dreamweaver-cc-2015/
But without installing an extension that works with MySQLi somehow you wouldn't be able to connect to a database on a php7 server. You could just downgrade your php version to 6 or below.