My Fitness app on my iPhone is not showing my steps or any other data

Which Apple Watch and iPhone do you use?

All devices are updated to the latest iOS and watchOS versions?

Update your iPhone or iPad - Apple Support

Update your Apple Watch - Apple Support

Does the Fitness App record other Health data, or did the Fitness App work in the past?

Did you check the settings for your Health App? Does it record data?

Use the Health app - Apple Support

Did you try to unpair the watch and connect it again to your iPhone?

Unpair and erase your Apple Watch - Apple Support

Just to make sure, your old iPhone is not paired anymore to your watch?