You have a few options here:
Put the SSD back in with your system connected to the internet (ideally via the Ethernet cable connection) lets see if we can get the internet recovery to run. When you boot up press and hold the Option-Command-R or Shift-Option-Command-R keys you should see a globe icon on your screen. Let the system run it may take some time! Depending on your link it could take a good hour to get to the install screen.
* Reference: [https://support.apple.com/en-us/HT201255|Startup key combinations for Mac]
The other approach is to create an OS installer thumb drive following this guide. Here we need to have the system running with the old drive so you can prep the thumb drive. Follow this guide: [https://www.macworld.com/article/3092900/macs/how-to-create-a-bootable-macos-sierra-installer-drive.html|How to create a bootable macOS Sierra installer drive]. Then put the SSD back in and then boot up with the installer drive.
If you get this: [https://www.startech.com/HDD/Adapters/USB-3-SATA-adapter-cable-with-UASP~USB3S2SAT3CB|Startech 2.5" SATA to USB adapter] it will allow you to hook up your SSD (or your HD) externally so you can leverage it for the OS installer and also migrate your files over.
* Reference: [https://support.apple.com/en-us/HT204350|Move your content to a new Mac (or new drive)]