Watch is paired, but not showing up in Watch app or under My Devices, no way to unpair it.

Hello. I have an Apple Watch Series 2 which was paired with my mom’s iPhone 8 (since then she changed phones and has an 11 Pro, but she set it up from the backup). I paired it about a year and a half ago, but she used it only once. Since then it has been collecting dust. Now I wanted to give it to my dad so I went to charge it and unpair it. But my mom’s Watch app doesn’t say there is a Watch connected (it’s only showing the “Pair a new Watch” screen). There’s also no Watch when going to her iCloud.com and going into Settings and My Devices. The watch thinks it’s paired, but the iPhone doesn’t.

What should I do?