Hi. Always a way
You're mixing two things I suppose: your sever and your domain name.
Apparently, you have a server on which your domain name is linked. Your server is basically a computer and you acces its hard-drive. Your old site must have been hosted in the "www/" directory, if you can access the root of your server, you should be able to see it.
To keep an ftp access to your server to upload files, you have 2 methods.
Method 1: do nothing. Yes, your server has an IP address and that's all you need. Replace the domain by the IP and it "may" work. It's not going to work if your server is a virtual server or if there's more than one domain linked to it.
Method 2 : at your registrar, setup a sub domain to access your ftp. For example, set a ftp.mydomain.com to point to your old server, and www.mydimain.com to Webflow.
You may need the help of a server admin to perform some of these tasks.