OP I have/had the same fear, can’t believe the ridiculous responses here assuming your DP must be contactable at all times! Lots of military personnel for example won’t be contactable for days/weeks/months on end.
What I did was work on independence as a priority in day to day life. I got a step my DS could move around himself and therefore reach most things (sink, fruit bowl, snacks). Doing it every day meant it didn’t make it a big deal about emergencies, he just thought he was learning how to do what I do, but in the back of my mind I was teaching him to survive in a worst case scenario! Yes, everything took ten times longer, but it worked and the bonus is he’s now very good at doing things for himself!
My biggest fear was we don’t have a latch on the front door so we lock the door with a key. Fine when I’m ok but not when not. Same thing - encouraged him to unlock it every day, so pull up the step to reach the keys, choose which key is right, unlock it. Obviously this took time and SO much patience, but eventually he got there. We also role played what to do if anything ever happened to me…use “hey siri” to FaceTime grandparents, use my phone to ring 999 etc. Also played lots of games (role playing doctors, school etc etc) where we’d have to give out our address and phone number so he learned without really noticing.
I purposefully got very friendly with neighbours and gave them a key but appreciate that’s not an option for you - I would get a key box at least.
None of us want to think of these things and it’s very unlikely something will happen to you, but there’s no harm in encouraging children to develop skills they can apply to emergency situations, without making a big deal out of them by just incorporating it into daily life!