Aliexpress
Here are coupons that you can use.
Latest coupon and price update (UTC):
Banggood
Banggood warehouse is located in Hong Kong.
Link: Banggood product page
Coupon (545.93 USD): Email5
Coupon (545.93 USD): bgshOOp10
(CouponsFromChina.com tip: coupon code has to be applied in shop’s cart after clicking “Proceed to checkout”. Click to copy code and open shop)
Exclusive Allowance Coupons: https://bit.ly/3xWgD9k
Old coupons (from the oldest to the newest): BG95T128W, plfonall, affiliate6, aff7off, bgssaff9, od7off, BGaDmITAd, 7bcdf9, BGBFAFF2019, BGCITYADS2019, BGMPB2020
(I don’t think these’re gonna work, but you can try ’em out anyway)
Price history and price alert
Check out price history chart (for all/most countries) for OnePlus 5T 8/128GB White.
Price records source(s): Gearbest, Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).