Yes, a bunker. Or perhaps a below ground basement.
I installed one of those nifty storm shelters that is slightly below ground level and back filled a bit around the part that sticks up. My home doesn't have a basement but an difficult to access crawl space in the event of inclement weather. We are in tornado alley here on the Ozark Plateau.
Here are the engineering challenges. Below ground, small vent for air that could easily accommodate a few wires without suffocating the shelter. Difficult to get a cell signal inside the shelter unless you hold your phone near the inbound or outbound vent. I cannot use a mast, or perhaps should not.
Now my station has had the ability of mobility for several years, and I can power the station via battery/solar. And I have not one, but two back up transmitters with the usual 3 meter antenna.
So along the cement flat roof I can in advance of a storm lay down horizontally my small wire antenna inside a plastic conduit and secured from wind. Easy.
What I do not have is a ground and wire coming inside. Easy to resolve by driving a rod and putting an 8 gauge bare copper wire in through the vent.
Am I overlooking anything? I haven't tested my station from this facility despite having it for three seasons now.
One last thing. How and what kind of external antenna should I mount for AM reception that is quite low profile? Remember, I need it to survive a strong wind and it cannot be on a mast.
Interesting setup. The cellphone is easy, cellphone antennas and boosters are cheap and you don't need a mast, there's several selections on Amazon. For your transmitter ground it seems to me you're already halfway there with your structure already partly underground, seems you could ground to it's framework or something, but that's just my initial thoughts.
A fiberglass whip for your transmitter antenna might be something to consider because they can handle the wind much better than a stainless steel ever could, I've always use 8' fiberglass whips for that reason.