Hi,
You have provided 2 links initially and mentioned that one works the other does not, I have checked and I see both do not work, so I’m confused.
Now you have provided another test site which I see that it works.
So if the only difference is the PHP version, please check the PHP error log and see if you have any errors there. If you have no errors, then I would recommend testing with the default theme with 3rd party plugins disabled, to make sure it is not caused by your theme or any plugin. If it does not work with the default theme and 3rd party plugins, then I’ll need a way to be able to debug it on your system.
If it works with the default theme and no plugins, then you need to start turning on plugins one by one and change your theme till it breaks, so you know which causes the issue. Do not forget to share your findings.
Thanks! ??