Watch is reset and needs update but can’t finish pairing process due to app saying phone needs updated even though phone is updated

Phone is up to date but Apple Watch app says phone software must be updated to pair with Watch. So my brother gave me his Apple Watch series 3, since he no longer uses it and would be great for me as a mom who doesn’t have her phone up her butt. Tried resetting Watch and turning off phone and turning it back on. Even tried resetting the network settings. Not sure what else to do. He said he did erase everything and set it back to factory settings.