You think your screen is too small ... Safari on my 6S is also defaulting to Desktop Site. However, looking at settings for Safari there's a toggle for "All Websites" under Request Desktop Site, so I can turn it off. You didn't even look there I guessI get it though. It's easy to forget that apps have settings that one has to go into Settings for rather than just some cog icon in the app itself.
Wow! At least my XS Max Safari is working as it used too.
My wife thinks I’m crazy but I hate the Facebook app. On the phone I use the mobile website, and on the iPad I use the website. I’m just used to it. I can’t find my way around the app.
Hope you can solve the phone issue. I do have the "All Websites" toggled off on phone and iPad.