Trump's tweet confirms high level concerns on both sides of the Pacific over the effect the trade ban will have on ZTE and its employees.
US officials are preparing for talks in Washington with China's top trade official Liu He to resolve an escalating trade dispute.
"But be cool, it will all work out!" he wrote. Unlike many other Chinese smartphone makers, ZTE's Android phones are also popular in the U.S, thanks to low-priced phones and savvy marketing ploys (the company's sponsored five National Basketball Association teams, including the Golden State Warriors). "This egregious behaviour can not be ignored".
"Imagine how the United States would feel if China had the power to crush one of our major corporations and make it go out of business", Reade said.
Then in February, the heads of every USA intelligence agency warned Congress that ZTE and the bigger Chinese smartphone manufacturer, Huawei, should not be trusted, since they may be forced to use their technology to spy on Americans by China's government.
The trade troubles threaten a technology sector that is increasingly intertwined with major players in the United States and China.
The US president also earlier this year blocked a deal that would have allowed a Singapore-based firm to acquire US chipmaker Qualcomm, claiming it would enable Huawei to set the pace the global rollout of 5G technology.
Other experts said Trump's policy reversal was unprecedented.
There is a risk of letting any company "beholden to foreign governments" inside the country's telecommunications infrastructure, he said.
Which brings us back to Trump. Lumentum Holdings and Acacia Communications Inc. sold key optical equipment.
ZTE suppliers including Acacia, Oclaro, Lumentum Holdings, Finisar, Inphi and Fabrinet, all fell sharply after the ban was announced. Oclaro, which earned 18 percent of its fiscal 2017 revenue from ZTE, fell 17 percent.