Apple sold 3.9 million iPhones in China in April, according to CINNO Research.

That's a 160% increase from March, when it sold 1.5 million smartphones, the Shanghai-based market research firm told CNBC. .

Apple's sell-in shipments totaled around 3 million in April, according to preliminary estimates by another research firm, IDC.

Sell-in refers to the number of iPhones Apple sold to its retail partners in China and can be used as a gauge for future demand.

In April, overall smartphone shipments in China rose over 94% versus March and reached 40.8 million, according to state-backed think tank, China Academy of Information and Communications Technology.

Revenue from sales of all Apple products via its official store on Tmall jumped nearly 40% month-on-month in April to $127.6 million, WPIC, an e-commerce tech and marketing firm that helps foreign brands sell in China, told CNBC.

CINNO Research said the iPhone SE accounted for 24% of all of Apple's 3.9 million iPhone sales in April, despite only going on sale later in the month.

IDC said the iPhone SE accounted for around 8% of shipments in April.

This is important because 5G devices are expected to account for 40% of total smartphone sales in China in 2020, according to Counterpoint Research

"The demand for 5G phones though is rising meteorically which could bring some headwinds to Apple's overall volumes in (the) coming months until a 5G iPhone is launched," Neil Shah, research director at Counterpoint Research, told CNBC