Upgraded phone, keeps crashing and restarting

I recently upgraded my phone to a refurbished S21 Ultra.  I like the phone, the camera is great.  3 days after using it, I had to install 7 updates to the phone.  I thought that was odd considering all refurbished phones are inspected etc. so I thought it was up to date.  After the new updates, the phone will restart itself almost once a day.  I will be using an app, such as Facebook or Instagram and it crashes and then restarts.  I found the feature in settings where you can turn on or off the phones capability to restart on its own if deemed necessary.  I turned that feature off and it will still restart after an app crash.  I've had Samsung phones for years and have been with Koodod for over 12 years.  I've purchased refurbished phones in the past but I've never had these issues before.  I'm on day 16 of when I purchased the phone online, even though I've had it for less time due to shipping.  I'm just wondering if it's worth returning and trying again.  Did I get a lemon of a phone ?