Greetings Svejdamios,
Thanks for visiting the Apple Support Communities! I understand you're missing Health and Activity data after removing your Apple Watch from the list of devices in Health.
I suggest ensuring Health is enabled under the iCloud settings of your iPhone. To do so, tap Settings > [your name] > iCloud, and then tap Health if it is showing turned off: Change your iCloud feature settings - Apple Support
Additionally, check for an earlier backup you may have: Back up your Apple Watch - Apple Support
Keep in mind, the backup may not have the data if your Apple Watch was not in range of your iPhone when it was unpaired:
"When you unpair your Apple Watch from your iPhone, your iPhone automatically creates a backup of your Apple Watch. Unpairing will erase all data from your Apple Watch. If your Apple Watch is unpaired while out of range of your iPhone, the backup might not have the latest data. When you're ready, you can pair your Apple Watch again and set it up from a backup."
Kind regards.