iPhone OS can't change font

Hello, I have set a default size and font in my online setting and this seems to be fine for my Mac OS desktop version. I'm using approximately a 16pt non-serif font which is fairly easy to read. However, on my iPhone, it is a small serif-font that is hard to read. I can't seem to change the font there. I do not see anywhere to change the font for this display. In addition, I can only choose either 13 or 16. That is a huge difference, actually, but am slowly getting used to the 16. However, in the serif font, it appears much smaller than 16. How do I fix this?

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