This something people don't understand. Fertility rate only affects the current generation. You cannot retroactively increase the fertility rate to fix the problem.

If the current generation has a 1.0 fertility rate, that means the next generation will be half as large. If said generation now doubles it's fertility rate to 2.0 it will only maintain the size of that generation, all the old people will age out and be gone all at once.

Now you might say, fine, let's fix it, 4.0 fertility rate, that will only double the size of the next generation, meaning it will only fix one generation of 1.0 fertility. You have stabilized the population, not increased it.

Even if you do this, you now run into a huge bottleneck: parents need to take care of their old parents and their large family all at once. You fixed the biological problem but you're now running into economic collapse.