Do you have overpasses in your area?
I used to live in Winnipeg ... flat as a pancake. The nearest hills were a long way off. But I had 3 overpasses in a row on the highway near where I lived, so I'd cycle back and forth, up and over and around those overpasses once or twice a week. I'd ride them as fast as I could, in fairly challenging gears.
Not ideal, of course, but better than nothing.
I've also heard that if you raise the front of your trainer you train the hill climbing muscles more, but I'm not sure if that's true or not. However, if you are stuck at home, you might consider doing things like commercial intervals for 30+ minutes ... watch your favourite TV show, ride comfortably while the show is on, ride as hard as you can when the commercial is on. And you might try raising the front of the trainer a bit.