The following process will not enable Sonarr to download the torrents as before. Instead, it is a workaround in order to Synology Download Station to do the actual search/downloading and Sonarr to do the rest; archiving, renaming etc.
Sonarr configuration should remain as is.
Visit https://showrss.info/ and create an account.
The final outcome should look something like the image of step 2.
After completing the setup, navigate to “My Feed” tab and copy the custom feed address that showrss will generate for you.
Then navigate to RSS Feeds section of Download Station and select the + sign on the top. Paste the URL copied from “My Feed” tab of showrss.info and tick the box “Automatically download all items”. Press Add.
Pray to the internet gods that everything works as expected!