Booting from stock slot doesn't work, as it was found in the topic before, I should have updated the first post to say this explicitly. For modern phone I would connect it to charger and leave for some time even if it doesn't display anything on screen, but not sure if this phone will charge without OS running, need someone who is more familiar to it to confirm. In the worst case you can use universal battery charger like this one. Sorry that it went this way for you.
It looks like you need a special cable (buy it or make it) to be able to bring it back to life without battery replacement. https://docs.google.com/document/d/1...FBR9VBa80/edit https://forum.xda-developers.com/sho....php?t=1077414 I haven't tested it on my own, though.