By placing a ramp (which reaches the height of the doorway threshold)
By placing a doormat (of height 2 cm or less which falls in the range of Xiaomi robot vacuum)
Method 1 - Ramp
Recycled rubber safety ramp OR Aluminum Modular Threshold Ramp OR Other material
Method 2 - Doormat
By placing a doormat on the longer edges of doorway threshold, for the robotic vacuum cleaner, a small step will be available to climb. Theoretically, this method should work for Xiaomi robot vacuum if doormat height less than 2cm and doorway threshold height is less than 4cm.
A low-pile, bathroom-style throw rug provided plenty of lift and traction to help the robo-vac cross over a threshold into another room. I tried project board (cardboard) and poster board over the thresholds, but it looked ugly and was always sliding around whenever the vacuum bumped into it.
The advantage of a throw rug is that it is inexpensive and can match your room so it doesn't look shabby until you get around to rolling it up after. Throw rugs also have a rubber backing so the ramp stays put and isn't always running away from you every time the vacuum bumps into it.