Elected Republicans have to support Trump, because the people who elected them support him.
Trump may be unpopular in general, but among Republican voters he is not just popular, but very popular. Like second historically only to Bush at the start of the second Gulf War. Those kinds of numbers.
The US is a country where the vast majority of elected officials now live in "safe" districts. This means the primary is a much more dangerous election to most Republicans than the general election. Heck, even in a tossup district, you still have to first win a primary where the only voters are Republicans. Attacking the head of your own party when his approval is north of 85% (which is likely higher than your name recognition, let alone your approval rate), is a great way to commit electoral suicide.
Effectively, with that kind of approval among the Republican electorate, an elected Republican's choices are support Trump, or be replaced by someone who supports Trump. That's it.