Recently my Readynas Ultra 6 Plus keeps rebooting randomly every 20min-1hour

And there is nothing in the log showing the power off, just a new power on?

A marginal power supply might cause that, especially the +5VSB (+5V Standby), which powers the on/off circuit.  That's a common issue with the older NAS models -- the power supply just doesn't last forever.  But, it is replaceable with a standard SFX format PSU, though you need a 24-pin extender cable and may need one or more SATA to Molex power adapters or Molex Y cables.  If you have a "spare" ATX12V supply, it can be connected externally to see if that fixes the problem before you invest in a replacement (though they aren't that expensive).

While the standard PSU is only 300W, I recommend at least a 350W replacement because the original has more 12V current capacity than a typical 300W.  I've used a lot of different versions, since many have come and gone from the market, and usually go with a 400W.

If you want the easiest/cleanest replacement, there is a vendor on eBay that has some that already have the needed cabling.