Thanks for the replies. They were most helpful.
For the curious, I know how my server became vulnerable. About three weeks ago I had a major network issue (it completely broke) and long story short I ended up removing my firewall (Ubiquiti USG), because it stopped working. I saw this as an opportunity to install pfSense onto my server as a VM. However, life, work, and health kept me from completing this in a timely manner. Now four days ago I got my server back online (was offline) in preparation to install pfSense, in the hope of upcoming free time, and yesterday is when my server was breached. Apparently, I never turned the firewall from my ISP modem back on.
To conclude, the rest of my devices on my network are fine, the network is secure again, IP address changed, and looking forward to combing through my server, connecting it back to the network and installing pfSense.