Is it actually cracked all the way through or just starting to split? If it's just the surface I might be inclined to scrape off the loose coating and run a seamless bead of white silicone along the length of that seam.
Ive found reinforcing the back with fiberglass can be very effective against further cracking/deterioration.
If the bottom strip comes off from the main back you could try removing it, squirt some clear adhesive into the split and then, using two battens, squeeze it together in a vice or between a couple of clamps until it has set.
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