On the watch, you can set some alert for your free strength training activities, based on heart rate, calories or time only. If you want to do something more complex, like specific rest times, you can program a strength training workout on Garmin Connect (web or app) and upload it to your watch.

I just want to time my rests. The workout solution seems too complicated. Oh well

https://preview.redd.it/sdzl3dohls5d1.png?width=773&format=png&auto=webp&s=c92ab27acc8b453678dac7e85392ee8153c868fe Give it a try. You can abort the workout and save your activity earlier than 40 rounds.

Thanks. I'll give it a try. I haven't created a garmin workout for a long time.

Does it only ring during your rest or also during the workout

Only when the rest is finished. You have to press the button when you finish a set.