USA commentators say the tone of the tweet is a dramatic shift for Mr Trump, who has consistently accused China of stealing USA jobs.
In March past year, ZTE paid almost $900 million in penalties for exporting USA technology to Iran and North Korea in violation of sanctions.
Representatives from both the USA and China - which boast the world's largest economies, with nominal GDPs of almost $19.4 trillion and approximately $12 trillion, respectively - held trade talks in early May.
Republican Senator Tom Cotton of Arkansas asked the group to raise their hands if they would recommend private American citizens use products or services made by Apple competitor Huawei or smartphone maker ZTE.
ZTE relies on U.S. companies such as Qualcomm and Intel.
Jobs in China are at risk, he said. The department banned shipments of USA technology to ZTE for seven years, saying the company had failed to reprimand employees who violated U.S. trade controls on Iran and North Korea.
The widening trade dispute between the world's two biggest economies has taken a toll on both sides.
In a second tweet on Sunday, Trump said past US trade talks with China posed a hurdle that he predicted the two countries would overcome.
It will also disrupt ZTE's overseas operations.
American technology companies are estimated to provide 25-30 percent of the components used in ZTE's equipment, which includes smartphones and gear to build telecommunications networks for telecom operators globally.
Last year, ZTE pleaded guilty in USA federal court to conspiring to sell us technology to Iran and North Korea, violating economic sanctions and trade export rules. ZTE and it's subsidiaries are banned from obtaining U.S. technology for seven years, putting the company's ability to license Android for their smartphone business and technologies and patents for their infrastructure business.
Other experts said Trump's policy reversal was unprecedented. Both sides had been expected to dig in for a fight over their trade imbalance at this week's talks.
"But be cool, it will all work out!" he wrote.
On Sunday, Trump appeared to ease the rhetoric, saying the countries "are working well together on trade".
The US imposed the penalty after discovering that Shenzhen-based ZTE, which had paid a $1.2 billion fine in the case, had failed to discipline employees involved and paid them bonuses instead.