Apple watch pairing issue

Hi ,

I am using Apple watch series 4 and previously it was paired with iphone 7. I recently shifted from iphone 7 to iphone 6 and now the watch is not pairing with iPhone it shows the Pairing option and even detects the watch the moment watch starts pairing the phone shows message about unpairing with watch and watch again shows the pairing screen. The iphone is running Ios 12.4.8 And watch is running latest watch os. Pls guide me how to pair it , i have even removed the watch from icloud but the issue persists.