When you say “html code”, do you mean an iframe (embedded code)?
If that’s what you meant, you should adjust the html code itself to mobile (for example using Media Query css).
I’m sorry. It’s out of the scope of this forum as it’s not about Velo but about an external code, and I can’t get into an unfamiliar external code (especially that I don’t know how it’s supposed to look).
assuming that the problem is with the width, I suggest, you’ll go over the html element and every where you see width in px you’ll add: max-width=100%;
Maybe it will help.
In case it does not work, do you have idea about my original question: how to make some mobile sites to load desktop version of itself on mobile browser?
You should know that many times it’s not a dedicated version for mobile but a responsive site and its style depends on the window dimensions and not on the device type.
Anyway, you can’t fake it. If it’s mobile - it’s mobile.
Do you maybe know is there a possibility to add button which will switch display from mobile to desktop view? I know that can be done through browser, but I need something straightforward so that my clients can just click on the button and see desktop version of page.