Xmas UK trip, bought pukka 4G/LTE TP-link WiFi dongle at Golden Computer Mall, Sham Shui Po.
Got 4G SIM from Vodafone shop in Bolton (don’t ask - but it’s right next to local 3 and EE shops!) Unlimited data cost £35 - we used over 40gb in <1 month.
most big airports have SIM card shops in terminals. T3 @ LHR has SIM card vending machine near luggage carousels.
For HK, 7-11 sells cheap 4G SIM card packages (eg HK$45 for basic China Mobile SIM).
Buy dongle from anywhere but phone company (!) Telstra dongle (Huawei) bought in Perth last year only allows use of Telstra SIM but charges A$3 per 1mb (FFS!) outside of Oz!