BEIJING—Chinese tech giant Huawei’s revenue fell 29.4 percent from a year earlier in the first half of 2021 as smartphones sales tumbled under U.S. sanctions imposed in a fight with Beijing over technology and security. Revenue declined to 320.4 billion yuan ($49.6 billion), according to figures released Friday, from 454 billion yuan ($70.2 billion) reported for the first half of 2020. Part of that was due to Huawei’s November sale of its low-cost Honor smartphone brand in hopes of reviving the unit by separating it from the sanctions on the parent company. Huawei Technologies Ltd. is struggling to hold onto market share after then-President Donald Trump cut off access in 2019 to U.S. technology and services including high-end processor chips and Google maps and music. Washington says Huawei is a security risk and might aid Chinese spying, which the company denies. Sales by Huawei’s consumer unit, which includes smartphones, tumbled …