In the configuration of your browser, you can usually configure that all websites must be using HTTPS at all times. If you have that enabled on your phone, and your website doesn’t support HTTPS, then you will not be able to access your site.
As for how to fix it, it’s very simple: you need to add HTTPS support to your site. And to do that, all you need to do is follow the steps in this article:
In short: write responsive CSS. This takes a little bit more effort than just adding a line of HTML to your page.
But by now, we’re discussing three fairly unrelated issues in a single topic, and it’s getting messy. I suggest we stick to the issues that are actually somewhat hosting related: the 403 error and the SSL. How you style your page and how you handle screen sizes is entirely a CSS code thing, and unrelated to hosting.