Also, if anyone has any tips...
Also, if anyone has any tips on how to maintain them better, and how to clean them properly, it would be much appreciated! I´m afraid I could have done something I wasn´t aware of to preserve them better and avoid those cracks :(
You would find it difficult for one to offer truly honest advice on preserving them, as there isn't none.
My advice would be not to let anyone over 80kg to use them. Hard ask in todays world.
As for repair. That's the easy part. Any good auto shop will do them or you could do it yourself with a fibreglass repair kit.
Experience has taught me to affix to small nails across the break within the fibreglass before you repair them as they'll give it extra strength.
Ball's right. There's not a lot one can do to prevent this type of failure except keep oversized individuals out of the chairs. They were intended to be maintenence-free, but not to last for eternity. In fact, it may not even turn out to be financially practical to repair them.