Oh how I feel your pain! I’m getting ready to recharge the battery in my nest doorbell, too, and came here for tips because my memory wasn’t quite fresh enough to remember how I achieved success last time; maybe what I found out will help you.
Picking up at the point of remounting the charged doorbell, look closely for the small, black, rubbery button on the back. It shouldn’t be flush with the battery pack; it needs to have popped up a bit to succeed in getting the doorbell remounted.
(This was the point for me where I spent a lot of time saying bad words because it wasn’t making sense; it was supposed to be simple!)
After a lot of googling, my fix was to take the tip of an X-acto knife and very carefully work it around the edge of the button until it popped up a titch, which was just enough to let me move on to the next step. (Just pressing the button in an attempt to get it to release and pop up does absolutely nothing, which I find confounding and the opposite of how reasonable people would expect it to behave.)
From my time searching, consensus is that this is a really cheap, yet critical, part (and not replaceable or repairable) that has caused no end of frustration and annoyance among lots of people. If your battery is still under warranty, though, Google will replace the entire unit and doesn’t seem inclined to make improvements to the components themselves to avoid this issue.
NB: in the same vein of small rubber parts, I once bought a Crockpot that had all of its feet, but one was loose in the box. I called to ask if I could get instructions on how to attach it; they told me not to bother, they’d send me a replacement and I didn’t need to return the first one. I eventually worked out how to put that little foot on, and then had two Crockpots!
Ok, back to doorbells. Once you’ve got that button up, try to remount again. When the instructions say to press the battery into the mounting plate, no need to be gentle; give it a good, solid push for the best chance of success.
Same thing for the last step of pressing down at the top; give it a steady, heavy push, and you should be rewarded with the magical sound telling you it’s locked into place.
I hope my solution ends up working for you, as well! (And if it does or doesn’t, it would be great to hear back how you fared.
Good luck!
