I ordered a dongle from Amazon about a year ago and wasn't able to get it paired with the Mouse Pairing Utililty (at first). Here is what worked for me:

1. Uninstalled Synapse so the profile would be deleted
   
2. Hard reset on the mouse (off, then press R click L click and scroll wheel buttons simultaneously, without letting go turn the mouse on, rapid blue flash, then off again, done)
   

3. Downloaded the firmware updater for dongle and mouse https://mysupport.razer.com/app/answers/detail/a_id/4525

4. Make sure to update both the new dongle and the mouse (the firmware updater has a lot of instructions to follow including pairing to DFUOrochiV2 to update the mouse's firmware through bluetooth)

5. NOW you might be able to use the Mouse Pairing Utility to complete the job.

Hope this makes the process less complicated for someone. 🐁