The users can not access the form tab when Facebook page is viewed over mobile phone

Also, they an only access the button link on their laptop/desktop, it can't be seen when they're accessing FB on their phone, can somebody please help? Thank you!

I took a look at your page as it is shown on mobile phones and I can see that the tab is indeed not shown.

There is the "more..." at the top however, that should give you more options, but which requires your visitors to be logged in first, which should show you the link to the tab with your form.

Do let us know how it goes.