Shopify Website not loading in certain iPhone models

We tried to do website speed optimisation for a Shopify store , but it is not loading in some iPhone models. Apparently it is not because of the file size or page size , we tried open the website URL in some iPhone but it just don't load the website at all and showing blank pages.

This shopify design has been customised before. From google pagespeed the scoring looks okay

Need help from someone who has experienced in fixing website not loading issue in specific iPhone , tested with few phones.

*Problematic*
1. Iphone 6+, ios 14
2. iphone 8 plus , ios 14.7.1
3. iphone 12, ios 14
4. Iphone 8 ios14.7.1

No problem
1. iphone 8, ios 14.7.1
2. iphone 7+ ios 14
3. iphone 7, ios 14.7
4. iphone 10 Max
5. iphone 8 plus , ios 14.7.1