V70: Open trunk with empty battery?

Hello and greetings, the following problem: The battery from my grandpa’s Volvo is empty, now stood around a little longer in winter, because it was more and longer in the hospital. The new one is already bought and I want to change it now. The battery is in the trunk. Since the battery is completely empty, my key no longer works – at least not the electronics that unlocks everything. So also no residual energy, the lamps in the dashboard no longer shine. Completely dead. I now assume that it is the battery, since I was driving in front of the MOnates and it has been in the garage since then. So I can only open the driver’s door manually. My trunk has no keyhole. Can’t be opened! In the normal instructions I have found nothing to help. Am I completely blind and missing something fundamental or has someone been sleeping in the construction? If you need the exact Verion number of the V70, please tell me, who The Volvo is already around 20 years old, but it was a lot in the garage and has only 40.000km, passed the last Tüv with flying colors and my grandpa doesn’t have any strength or nerves for another purchase – so if a solution could be found here – that would be great. I can’t just drive into the workshop. Thank you very much for the help!