There was a patch released recently that causes previous versions of Daedalus to stop syncing:
I’m not sure what the status of the Linux version is, but you could try making sure it’s updated to the latest version and see if that fixes the issue.
You can’t put your 12 word Daedalus phrase straight into Yoroi. You need to create a new Yoroi wallet with a 15 word phrase, then click the Daedalus logo at the top right of Yoroi and choose to import the funds from Daedalus. This transfers all the ADA out of your Daedalus wallet into your new Yoroi wallet.
Edit: Here’s a video on how to import Daedalus to Yoroi