Why is Trump so unpopular, and what can he do?
The main reason Donald Trump is so incredibly unpopular  is because he does things the "Breitbart Way", and the Breitbart Way does not work and is only popular with a fixed segment of the U.S.
The fact is that Trump's signature policies - when seen from a very high altitude - are very popular. Most Americans want to reduce illegal immigration and immigration in general. Almost all don't want to let terrorists into the USA. Many or most oppose globalism. (And, as can be seen from the thousands of posts here since 2002, I agree).
The problem is how Trump does things: he invariably does things the Breitbart Way.
For a living example of the Breitbart Way, go to Breitbart or a similar site and leave the preceding three paragraphs of this post as a comment (or tweet a condensed version to a Trump supporter). You'll soon be awash in the Breitbart Way.
Instead of attempting to intellectually engage you and attempting in vain to show you wrong, those following the Breitbart Way will call you names, jump to false conclusions, engage in misplaced "whataboutism", engage in "just you wait!"-ism, show intense Dunning-Kruger, and in general be vile, anti-thinking, low-integrity, deranged idiots. They won't be able to tell the difference between what you want and how you want to go about getting it. They'll show that they think that illegal immigration has to be opposed in the same faux-tough way that Trump and Breitbart do it, even though their way won't work, is just for show, and eclipses smarter ways that would work. Even though you made it clear that you want to reduce illegal immigration, those following the Breitbart Way will accuse you of supporting it. And, they'll do that to enable someone who's intellectually incapable of doing what's necessary to reduce illegal immigration.
To make it clear, the Breitbart Way is the same as the Ann Coulter Way, the Gateway Pundit Way, and all the rest. The way they do things is great for their web traffic, but - as Trump's popularity shows - it completely fails at anything else.
Trump consistently undercuts the goals he pretends to have by dealing with these issues in the dumbest, most deranged ways possible. Trump consistently has to do shtick. He always has to play to his base and stay in his comfort zone. Trump has consistently run from debate rather than trying to engage his opponents and show them wrong.
And, most Americans - two-thirds or so - reject his shtick.
Trump is - to put it in polite terms - polluting attempts to reduce mass and illegal immigration, reduce establishment media deception, and undercut globalism. If Trump were smart, sane, and patriotic he'd make smart arguments for once. Smart arguments - such as those on the talking points page could easily increase his popularity and undercut those he and his supporters pretend to oppose. Trump and his supporters - as shown all during the last two plus years - lack the smarts, the sanity, and the patriotism to change how they do things even though it would help them achieve the goals they claim to want.
All of this is tragic for the U.S. We have a president who's colluding with Nancy Pelosi on amnesty, who's continued Barack Obama's refugees agenda, who's endangered the U.S. by his own admission, and who - after he leaves office - will have done nothing to reduce illegal immigration (as whatever of Trump Wall that was built is torn down).
The solution for Trump is to finally act like president of the entire U.S. and to finally make smart arguments that will undercut and marginalize both the cultural far-left and economic libertarians. He can reduce illegal immigration for real if he stopped doing things in the Breitbart Way (and the Coulter Way, and the Gateway Pundit Way). Even though it would be in his best interest - not to mention the actually-important U.S.'s best interest - to do that, Trump lacks the smarts, the sanity, and the patriotism to help the U.S.
 After his first year, Trump is more disliked than any president in the last seven decades of polling.