Refurbished devices, specifically from Apple, should be essentially flawless. Any damage or faulty parts are replaced as part of the refurb process. Some people swear by Certified Refurbished, for this alone, as the device is less likely to have issues straight out of the box. More on reddit.com

Refurbished products from Apple store are practically brand new More on reddit.com

Which one is yours?

GPS + Cellular versions:
A2725 40mm, A2724 44mm (International);
A2726 40mm, A2727 44mm (USA, LATAM, Canada);
A2855 40mm, A2856 44mm (China, Hong Kong)

Visible (from the internet, not sure if it is accurate):
LTE / 5G (DSS) Bands: 2, 4, 13.
5G Bands: n261 (2800 GHz), n260 (3900 GHz).

Maybe... you have a cellular version that does not work on Visible?

For instance, China uses (from internet search):
4G Bands: B3 (1800 MHz), B8 (900 MHz), B40 (2300 MHz), B41 (2500 MHz)
5G Bands: n78 (3500 MHz)

not sure if this could be the issue, but I hope it helps...

If you managed to pair it to your iPhone, I don't see how this could be related to activation lock, maybe someone else can share some light.

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The patent on the blood oxygen sensor expires in 2028, so maybe try to make it last until then if blood oxygen is important to you. Maybe I just don't know how to use it, but I don't see the big deal about the blood oxygen sensor. Getting a blood oxygen measurement involves opening the Blood Oxygen app to consciously take a reading, and then I don't know what I'm supposed to do with the result. While at the hospital earlier, I was hooked up to a machine that continuously took my blood oxygen reading. For fun, I tried breathing a lot to try to raise it, and was able to get it all the way up to 100%. I also compared the hospital's device to my Apple Watch, and they both seemed to match. More on reddit.com