Aliexpress
Here are coupons that you can use.
Latest coupon and price update (UTC):
Banggood
Banggood warehouse is located in China.
Link: Banggood product page
Coupon (17.84 USD): BG75597c
Coupon (19.10 USD): Cuponation9
Coupon (19.73 USD): BG00c906
(CouponsFromChina.com tip: this code has to be applied in your 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): BGTYSY956, BG10OFFAFF1, BANGGOODSD, 6GSGIT, BGBF20off2, BGTYPS956, 00c906, BGXPJA135, BGHealth10D, 2022ACTIESNL, BGAlerabat1, BGHB218, BGXPMA145, BGMAYB241, 2203ActiesNL, BGHECARE1, BG4ee866, BGHU2285, BGdf740b, BG9f2ee2
(I don’t think these’re gonna work, but you can try ’em out anyway)
Price history and price alert
Here is price history (for all/most countries) for Xiaomi Mijia XMWSDJ04MMC Thermometer Hygrometer.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).