This definitely works for crossposting from Mastodon to Twitter. Make sure to check manually and give it plenty of time to post. You may want to turn on email notifications when the applet is run so you know it's triggered. (Then you can turn it back off)
https://medium.com/@gigap/not-working-for-me-have-the-proper-atom-feed-url-but-the-ifttt-action-is-not-triggered-even-on-9d1c688061ed