Greetings from Syncfusion.
We have checked the reported issue "Combo box not working in iOS" using provided code example and we are afraid that we are not able to reproduce the issue at our end. We have attached the tested sample for your reference. Please find it from the below link.
So, can you please provide the modified sample based on your application along with replication procedure. This will be helpful for us to investigate further and provide you a better solution at the earliest.
Muneesh Kumar G