Try side android P and check if its happening there.. if it happens there. .then its a hardware issue... but frequent rebooting is normally a software issue..
It's a Wi-Fi related problem. Try turning off the Wi-fi on the phone and see if it happens. I had the same problems, but have just been using the phone for a week with no Wi-Fi and had no reboots at all. I also noticed that in weak Wi-Fi areas it's a lot worse - basically every time the phone requests data through Wi-fi (like when you open an app) it would restart.
I've told HMD Global directly about this problem (I'm a tech journalist) and I'm going to keep on at them for a response - I'm also going to send this thread over to them as this is obviously a real problem affecting lots of people.