I bought a new iphone and data was transferred from old one. However, apple watch appear as not connected to iphone

See if the watch is still paired with your old iPhone. If so, follow these steps to unpair it and then pair it anew

If you have the paired iPhone, unpair your Apple Watch:

1. Open the Watch app on your iPhone, tap the My Watch tab, then tap All Watches at the top of the screen.
2. Tap the "i" button  next to the watch that you want to unpair, then tap Unpair Apple Watch.
3. Tap again to confirm, then wait for the watch to unpair.
4. After Apple Watch unpairs, you can pair again.