Relevant section on Wikipedia:

The government may not criminally punish immigrants based on speech that would be protected if said by a citizen.[83] On entry across borders, the government may bar non-citizens from the United States based on their speech, even if that speech would have been protected if said by a citizen.[84] Speech rules as to deportation, on the other hand, are unclear.[85] Lower courts are divided on the question, while the leading cases on the subject are from the Red Scare.

https://en.wikipedia.org/wiki/United_States_free_speech_exceptions

Regarding the protections against illegal searches, I think all bets are off, even for US citizens.

https://en.m.wikipedia.org/wiki/Border_search_exception

Federal law allows certain federal agents to conduct search and seizures within 100 miles (160 km) of the border into the interior of the United States.[5] The Supreme Court has clearly and repeatedly confirmed that the border search exception applies within 100 miles (160 km) of the border of the United States

If any agent of the government tells you otherwise, you must educate them and enforce the free exercise of your own rights.

How exactly does one enforce the free exercise of their rights?

Remember that agents of the government are often armed and generally don't like being told what to do.

Thanks for the correction