Hello,
HikaSerial is a Joomla component ; it does not have any knowledge of the "remote" device and have one single workflow for consumption.
Only global setting, pack settings and "consumer" plugins can affect that workflow and define its rules (if guest are allowed, if a linked serial can be consume, etc).
You can say it's a bug in the code and I can answer you that there is no one single line of code which will change the behavior of HikaSerial if you're using a desktop or smartphone browser.
But the cookies and the session can be different regarding Joomla and the other plugins you have installed (like a cookie consent).
Even if you're browser as a guest ; the form submission is using a Joomla token, which requires a session.
Without cookies, you don't have a session and you can't consume a serial (even with guest).
It's one of many possibilities.
And if you can provide the screenshot and the other elements I asked you previously, I would be able to confirm some other thoughts.
Your account in our website does not own an HIkaSerial licence and your website is not link to an HikaSerial subscription.
If you want to send us admin access (via the contact us form only) ; please also indicates your subscription licence so we can link your website with it.
Regards,