It is surprising that Apple struggles with XenForo... when PC and Android have no problem with it
The only thing I can suggest is to try loading a different browser (Firefox or Chrome). Navigate to the site (using the same phone) but with the different browser. If the site works fine using the different browser, you will know that the problem lies with Safari. There are reports of Apple users having the same problem on different websites, and Apple suggests certain steps for deleting your cache and your browser extensions. It is difficult for me to troubleshoot remotely when I can't replicate the problem