Hey Berk,
Obviously your goal is to isolate and reproduce the problem so that you can fix it. You’d have the client go through the same basic steps regarding network, wifi enable/disable, disable any VPN, etc to try to identify it.
Then you’d try to recreate that locally or on a service like browserstack.
Either way, you want to see what errors are produced in the browser’s console log, and that process is different based on browser and device.
If I’m right that it’s a jQuery load failure, you could also try force-loading jQuery in the site-wide before-head custom code area using a CDN script URL, make sure it’s the same version Webflow is using. That’s not ideal and might have other impacts, but it could be a bandaid here for certain issues.