Failsafe for when wi-fi goes out?

So this is a more conceptual post. I had a moment of panic last night. I have a headless Pi 3 running home assistant with z-wave door sensors and an Aeotec Siren. It’s connected to my router with an ethernet cable. I have a simple security automation set up. Using a Boolean input, I can enable or disable the automation: door opens, siren goes off.

Well, I went out last night after my kid was put to bed, set it to armed after closing the front door. A few hours later, my wife texts me that the wi-fi went out. I asked her to reset the router but she had already gone to sleep! Without wi-fi my LAN IP address shortcut on my phone is useless! Luckily the wi-fi was fine when I got home. But it got me thinking that I should have a contingency plan in case this actually happens. I don’t want to wake up a toddler or give my wife a burglar scare. My z-wave siren doesn’t care about a lack of wi-fi!

Have any of you designed contingencies around this problem? Some of my rough draft ideas are:

  • Connect to my Raspberry Pi 3 through bluetooth? It is bluetooth-enabled but can I reach the home assistant front end through it?
  • I just purchased some 433 MhZ RF outlets. I could power my Pi through one and hide a spare remote behind a physical lock in the shed or crawlspace. This is admittedly a crude, brute force method, but the siren can’t get the signal if my Pi is off.
  • Put one of these RF plugs on my router, possibly creating an automation that turns off power to the wi-fi, waits 10+ seconds, then turns it back on. Of course, there would have a to be some kind of wi-fi sensor so that Home Assistant could do this without me, because I can’t access the Pi if I don’t have wi-fi.

My Pi is (theoreticaly) currently accessible only on my LAN. But if I have my external IP address, what are the barriers to entry to get into my Pi from my phone’s cellular internet connection?

What are your ideas?